Revolut, Grab, Gojek, Tinder, WhatsApp, and many more are all successful that offer a mobile-first solution, with either no web/desktop client or just as a an additional client (usually with fewer features, and/or still requiring access to a smartphone).
> Also "any chance of any form of adoption" is a bit overstatment. I still use a dumbphone
Of course it's possible; but depending on what your interests in life are you will pay a price, and in practice for the vast majority of people the price is too large to use a non-Android/iOS compatible device.
> many apps have web alternatives here, or alternative GPLed clients for Linux (that includes whatsapp, apparently), that can be made native on pinephone.
Unless they somehow hacked the encryption, you're still going to need a connection to the phone's WhatsApp client.
Apologies if I sound a bit naive, but what would be there to "hack"?
WhatsApp clients are available for many platforms, whatever encryption they might be using can easily be figured out by decompiling the code, and if they are using a key on the client side to do any encryption, that key is available for extraction from the distributed client too.
Basically, my question is what can a closed source downloadable client do to protect the encryption it uses to connect to a public network?
For regular companies, if they want to shoot themselves in the foot by not being on the web, they're welcome. It's not such a huge issue as it would be with government for example.
Also "any chance of any form of adoption" is a bit overstatment. I still use a dumbphone, and if I migrated to pinephone, lack of the kind of apps you mention would certainly not concern me. Even then, many apps have web alternatives here, or alternative GPLed clients for Linux (that includes whatsapp, apparently), that can be made native on pinephone.