Imagine being a Mac Pro buyer who spent $50,000 in 2019 on the top-tier configuration, only for Apple to announce its migration to Apple Silicon a year later. Now imagine a company that invested in 10, 20, or even 50 of those Mac Pros—would you buy Apple again?
Software running on those Macs isn't going to suddenly stop working. It would likely run for another 4-5 years without any update, but companies making such large investments would also likely not keep using "legacy" computers for a decade. They'd eventually upgrade to keep pace.