Sure, there could be cases where you have absolutely no alternatives to do something really specific, but in those cases I'd first invite you to reconsider whether what you're attempting to do actually needs to be a web app. Of course, people's thoughts will probably be split there.
However, the bottom line is still this - large sites load slower and cost more to load, they consume more battery and CPU, which on lower end devices could lead to system instability. If users don't outright leave your bloated solution (i.e. if they're forced to use it because of network effect) then in the case of any non-optional conditions (slow or unstable networks, low end devices, expensive data) their experience will be miserable.
Maybe have your IM client be a downloadable app instead? Better yet, why not have it be a native app instead of a bundled browser app, for excellent file size, small attack surface and great performance, while conforming to the OS look and feel?
Alternatively, why not make your IM client have a lightweight browser version that doesn't try to do everything under the sun? Personally, i feel like the answer is not to make browsers do more, but to try to do less yourself.
To that end, utilize server side rendering and pre-rendering with hydration when needed. Split some bundles and shake some trees. Compress as much as possible and use boring, common fonts while preferring local versions if available. Avoid videos in most cases and use smaller images, or vector graphics. The web can be a simple, beautiful and fast place if we make it one.
