Instances can themselves handle links agnostically, and clients are able to access that via the API.
Using the autocomplete in the webUI can mess things up, as it is from back when community mentions weren't even turned clickable. The link is then an explicit markdown link to the user, and may not be correctly clickable in whatever client others are using, but may instead open an off-instance page in a new tab/browser app.
Even if you link to a post using its local url and ID, a client can, and some already do, open that link in-client from your instance. But this is something that still needs to become more widely implemented.
The "manual" way to open off-instance links locally is to enter their full source url into search on your instance, which should give you their local version as the sole result.
User mentions do not need to be links, same as communities. Just write @username@instance.com and nearly all clients will correctly make it clickable, and Lemmy will also detect and notify the user about the mention.
I don't use any client besides the web-ui. There is no reason for the webui to not impiment obvious features found in multiple client apps, but yes, you've hit the nail on the head as to a desirable version of this behavior.
There is one big reason: it requires time and effort.
The vanilla web-UI is inferior to other clients in a variety of ways, and any feature requires that someone somewhere get it done.
You could make an endless list of things that are "no-brainer" features for someone to add, but the development still requires that someone somewhere go down that list, one item at a time.
For example, Lemmy only recently got the ability to show the real number of subscribers that communities have, instead of showing the number of subs from the instance you were looking at that community from. That's nice, but each update can only come with a subset of the endless number of obvious improvements that could be made.
No client currently does everything that the Lemmy server can technically be made to do with the currently implemented API calls. But the solution to agnostic links already exists. Any instance, when handed the url to a post or comment, no matter what instance it is for, is able to turn that into the corresponding local url/ID, the work to make use of that to make all links work everywhere simply hasn't been put in yet.
Not that long ago just mentioning communities and users was a pain, but that's mostly solved now. Posts and comments will be too. Eventually.
You literally dismiss the necessity for any effort in your first paragraph and claim extentions/apps are somehow preferable, then in your last two paragraphs call out that the function is already present on the servers and the servers more feature-complete than the apps.
Pick a narrative. Letting the apps represent the more useful/enjoyable experience is a big reason the eventual API rugpull on reddit was such a shit-show. If your lesson-learned was "more app, more gooder"/"everyone should constantly reinvent the wheel in pursuit of fun and profit, so long as no one asks anything of the original devs", you haven't been paying attention.
When devs can no longer be bothered to impliment obvious features, its time to fork, not present a new compatability layer that you control as the solution. Its both a shitty user-experience, and an almost impossible path to fun and profit for any dev who has no control over the original code. Then you get dozens of others having to reinvent the wheel every time the original dev DOES impliment anything that breaks compatability with their apps, all in pursuit of dimes they will likely never see.
I, and most users, are here to participate in discussions, not promote the LateStageCapitalism rate-race for the next disposable-and-planned-obselecence-baked-in-to-its-very-nature digital widget.
Lemmy doesn't have the development resources to Thanos snap itself into a feature-complete state overnight.
Forking would make that worse.
I can smell the entitlement of "when devs can't be bothered to do the obvious" straight through my screen. We simply do not get to sit here and come up with things that are "so obvious they should be done already". That's not how reality works. The wheel didn't exist until it was invented, and then built by someone.
I'm only explaining that the solution exists and is currently held by back by the fact that development isn't done by materializing code into existence with an infinity gauntlet!
The fact that some client devs implement new API features before the vanilla webUI does (which btw is a separate thing from the lemmy server, and itself merely one client for it, albeit the default) isn't unusual, or indicative of some massive fundamental problem in how the project is run.
Read up. I stated I would be refraining from speaking to the difficulty of implimentation as I do not have my laptop with me.
I've written apps/extentions for personal use. I would rather contribute to an existing code-base like Lemmy, but I acknowleged the fact I am in little position to do so, for the next two weeks in fact.
Entitlement, really? How is your passing the buck to extention/app devs different from my requesting a feature be implimented in the single location where it will stay fixed, saving time and effort for the devs, including for app and extention devs, going forward?
Where is your suggested solution that could be implimented, beyond suggesting that apps and extentions have this solved for their users, and we should leave it at that?
I've bought multiple such apps, and found the webUI preferable, so I'mma throw my future suggestions, requests, and yes, financial contributions, as well as any code I write if that turns out to be necessary on my part, at the single project that is already doing so many other things I've come to enjoy.
I'm not passing the buck anywhere, I'm saying the lemmy server can do this but the default lemmy webUI doesn't make use of it. At least not yet.
The problem isn't being solved by the clients and apps. It's solved by lemmy.
Some clients have just made use of it sooner than others, and the vanilla webUI is one of the ones that hasn't yet. But the feature itself IS IN LEMMY ITSELF.