It's not a given; it's a rumor.
The only likely reason I could see for Apple going back to "legacy ports" (from Apple's perspective of course), is if they have some limitation in Apple Silicon that means they can't offer 4 TB3/USB4 ports on the next machines. That would be a terrible admission of design failure. If they fixed it next year, would they then replace HMDI/SD card slots with the TB3 ports again? Flip-flopping on the design would be even worse...
It's certainly conceivable that Apple has conducted customer and market surveys and found that they are losing MacBook Pro sales due to it missing these ports, but most people who need these (myself included) have just moved on and have USB-C adapters (or docks / cables) for all of these.
Apple is quite considered in its design changes, and I can't think of another example where they've gone back from a "future-looking" standard, like TB3 to an older one.
I agree that HDMI and SD card slots are still useful to many people, but I just don't see Apple back-tracking unless they are forced to.
I guess we'll see! I may be completely wrong...