Well, it all depends on what the "client" "wants".
The problem often lies with the client tantrums.
I was more talking about critical online services which have a duty of interop with Small Tech and must not force users into Big Tech engines only, that to keep the door open to alternatives of reasonable "size", and at the same time to lower significantly exit costs.
But that's smashing open doors, most are aware of those issues here on HN. The real issue is implementation: it has to happen in a regulatory framework, laws dealing with discrimination, and my lawer has been looking into other legal leverages (yeah, I have an open conflict with my administration). The regulatory framework is here, but lobby-ied heavily by Big Tech, making it ineffective and pointless.
A bit like what they have in the US with the "utilities" which then have much more regulatory duties than "non utilities". You also have anti-trust stuff: dominant corpos are much more tied to regulatory constraints, that to let alternatives to be viable (well, that does not seems to work very well in the US). In my country, anti-trust ultra-aggressive regulations did wonders in the telecom industry.
You can still have a web app with a rich GUI, an anonymous simple and stable in time public HTTP-based middleware protocol (which allows to develop Big Tech independent rich GUIs), etc, until the core functions are provided to noscript/basic (x)html browsers (if one thing must work, it is this one).