Proposals had been made to change Russia's nuclear doctrine to allow for attacking any non-nuclear state that had the participation or support of a nuclear state, Putin said.
That's the thing though, as long as common sense still exists and neither party is willing to use nukes aside from threats, they still have to fight the US with all the other means. That means that the US will have to cope with multiple fronts at once as the war spreads out in the world, and there must be a tipping point where they just can't handle them all anymore.
i get that viewpoint but isnt there a moment where they go like am i testing my weapons and make money, or am i just training my enemy and improve their capabilities?
I think the most likely response from Putin would be to arm the Houthis/Ansarallah with advanced anti ship missiles, along with selling Iran advanced weaponry. Russia has lots of these missiles in storage, Ukraine has no navy that can be targeted with them. If NATO wants to open Pandora's box and give advanced weaponry for long range strikes to a proxy in Ukraine, Russia can absolutely do the same thing in the Middle East.
So far the only actual red line that was laid out by Russia was Ukraine joining NATO, and when that line was crossed we got the war. Pretty much all the other red lines have come from the west, like when the US said they wouldn't send missiles, tanks, F16s, and so on because it would be a dangerous escalation. Then the west would cross their own red lines, and say that see nothing bad happened.
Russia's statement on long range strikes into Russia is the second red line that was actually stated by Russia, and I don't think it's a bluff either.