Their exact wording is as follows:
“So what’s the timeline for this transition? Well, for developers it begins this week with the valuable information delivered at this conference as well as applying for the Quick Start Program. And for the customers, we expect to ship our first Mac with Apple silicon by the end of this year, and we expect the transition to take about two years. We plan to continue to support and release new versions of macOS for Intel-based Macs for years to come.” —
Tim Cook, WWDC 2020 keynote
Trust and believe that they meticulously thought out every word there. Given the RAM and I/O limitations on the M1 SoC, I can’t
really fault someone for thinking it bodes poorly for replacements of high-end Macs within two years. My personal expectation is that they’re resolved soon and every Mac does get replaced, but Apple seems pretty careful to avoid saying or implying that the transition will necessarily include all Macs.
As for macOS updates for Intel-based Macs, they left it at a nebulous “for years to come.” That could put the EOL date as early as June 22, 2022, or two years from the day those words left Tim Cook’s mouth. I suspect that regular macOS updates will be released through 2025 or so, and security updates for the usual three years thereafter…but again, Apple covered their bases to give them a great deal of leeway here.