I'm not TeMPOraL, but to my understanding:

Chrome is a duopoly, and Firefox tends to get forced in the same direction as Chrome over time -- it's not powerful enough to push the web as a whole in a different direction.

If Chrome was not a duopoly, this would not be a problem. There's nothing fundamentally wrong with having a browser that's highly optimized for nontechnical users. However, because Chrome has such control, pushing technical users out of Chrome has the effect of pushing technical users out of the web entirely. By putting itself into a dominant market position, Chrome has put itself in the (unenviable) position of needing to be everything for everyone.

In other words, if Chrome doesn't support power users, and if Firefox doesn't have the guts to refuse to make the changes they advocate for, then power users have no where to go. This is why monopolies and duopolies are bad even if the companies behind them are good. Its completely unrealistic for a single product to be everything for everyone, so single products by their very nature exclude certain users.

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So who cares if power users abandon the web?

This is a problem because historically, power users have been incredibly important for the web's development. They've been important not just in the sense that they've pushed the web forward technically, but also because the web was designed to be a democratizing medium. The web is designed to be a publishing platform where people can build weird things without asking anyone's permission.

Even non-technical users benefit from this -- it's in their best interest to have lots of providers providing lots of solutions for publishing because (repeating what I said above) no one platform can be everything for everyone. A non-technical web that is optimized for ordinary people to the exclusion of power users is going to be less creative, less interesting, and less useful even for ordinary people.

Whether or not FTP in particular is critical to that -- I don't know. I don't know enough about FTP to make a strong argument on that. I will say I have used FTP a lot as a consumer, especially when downloading packages, and I have used FTP on internal networks. I'm not an expert, but the arguments package maintainers make about why SSL is unimportant to FTP ring true to me. My understanding is that package signing prevents modification attacks, and that looking at request sizes is enough to identify resources even with SSL.

As a policy, deprecating URL types that don't support encryption seems at first glance to be a really dumb move, and I'm not sure the security arguments justify it. But I'm not an expert.

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I'm on Firefox, and I'm hopeful that Firefox is going to see a resurgence of usage pretty soon, particularly among power users. If Firefox refuses to deprecate FTP, I'm not so worried, since honestly I think it's probably in everyone's best interest for Chrome to do a few dumb things that make more power-users jump ship to Firefox. A lot of power users aren't willing to make the switch on ideological grounds; Chrome as a browser needs to get much, much worse to encourage them to use competitors.

Thankfully, it appears that (especially recently) the Chrome dev team has been thinking the same thing.