If the browser is computationally expensive abstraction, so is the various .NET SDKs, the OS, custom compiler and the higher language of your choice. Yes there were days were an game like prince of persia could be fit in to the memory of apple IIe and all of it including the sound graphics, mechanics and the asset was less than 1.1 MB !
However the effort required to write such efficient code, hand optimise compiler output is considerable not to mention very few developers will be able to do it.
Unless your domain requires high performance(with wasm and WebGL this will also be reduced) or something niche a browser cannot not currently provide it no longer make sense to develop desktop applications. The native application is too much hassle and security risk for the end user compared to a browser app and is worth the trade-off in performance for vast majority of usescases.
While the browser security sandboxes have its issues, I don't want go back to the days of an native applications constantly screwing my registry, launch processes , add unrelated malware and billion toolbars to your browser ( java installers anyone ?) .
Till late 2000's Every few months I would expect to do reinstall the entire OS (esp Windows and occasionally OS X) because of this kind of shareware / malware nonsense native apps used to pull. While tech savy users avoid most of this pitfalls maintaining the extended family's systems was constant pain. Today setting a chromebook or surface( Default S mode enabled) and installing an ad blocker is all i need to do , those systems are clean for years.
I do not think giving effectively root access and hoping that that installing application will not abuse is not a better model than a browser app. It is not just small players who pull this kind abuse either, Adobe CC suite runs like 5 launch processes and messes up the registry even today. The browser performance hit is more than worth not having to deal with that
Also just on performance from a different point of view, desktop apps made my actually system slower, you would notice this on fresh install of the OS, your system will be super fast , then over few weeks it will slow down, From antivirus to every application you added , they all hogging more of my system resources than browser apps do today.