Don't be that pessimistic, most users had to install Reddit, Twitter and TikTok apps. In the 2010s, grassroots chain emails and Facebook posts with guides to setting up WhatsApp went viral among boomers in my country, touting it as "free SMS". (Facebook camnot legally describe it as "free SMS" but they didn't bother correcting anyone of course.) The fediverse experience is already quite OK if you have a dedicated client but the problem is that not everyone does, which is why we need browser support; people are tired of "wOrKs bEtTeR iN ThE aPp" even if it's true this time. A dedicated URL scheme will automatically associate Fediverse links with any appropriate installed web/local apps. There are still other issues such as hit-and-miss cross-fedi-platform compatibility, no API for retrieving the list of federated instances and lack of appropriate error messages if the source and/or destination instance block each other.

Grandmas can easily install Cookie Clicker and Angry Bird and DownloadMoreRam and anything else they find on the internet with quite an ease. Are you telling me the average internet person is noticeably less capable than a grandma?