I don't think people are worried about storing hundreds of Signal instances, this isn't a photo backup.
The concerns are bloat, optimisation, and memory usage.
That's the point. The storage is a bad metric. While it might indicate poor performance, it's not a direct indication of poor performance. The bloat and optimization comes from the usage of Electron. And people use Electron because it's far easier to make cross-platform deployments for Web and desktop using a framework like Electron. Show me the QT/JavaFX app that mimics Signal and we can compare the cost to develop it. Electron isn't the best choice for memory usage and reducing bloat, but it's the best choice for quick development (in my opinion but also proven out by the market share it has)
there is an inevitable cost to written code though, it simply cannot be computed away. In this case the cost is just a shitty application with an even shittier user experience.