Very true. Here's a possible scenario. Say an individual, 1 person business buys the 2019 MP. They spend $10,000 to get a decently specced machine, planning to keep it for 9-10 years. The room for upgrades of ram, CPU, graphics and storage make that very possible. Now Apple switches to ARM based MPs in 2021. After that they drop support for the '19 MP in 2023, about 4 years after this individual bought one. The value (resale) of that MP drops precipitously. No one wants them anymore so he can't get much back from his initial investment. He can't upgrade to the most current Logic Pro, FCP X or XCode anymore. Isn't that one Professional Apple customer that will be lost forever. Wouldn't you be if you bought a MP right now and this happens 3 years later ? This is why I think they'll keep supporting Intel Mac Pros for much longer than 4-5 years past late 2019. Even big video production houses that spend hundreds of thousands of dollars will not be very happy to have to upgrade all their MPs that quickly. They could certainly switch to Premiere Pro or Davinci Resolve and use a PC workstation instead.
After Apple put so much time and research into reviving the MP tower, now upgradable again, I don't think they will do something like this to make Pros give up on them for the long term. There's no reason they have to. They might keep developing an Intel version of macOS specifically for the 2019 Mac Pro even if all other Macs switch to ARM. macProOS. That would even be great for this community. They've now got a unique OS specifically for iPad Pro. Why not for the Mac Pro too ?