I think these models will not be replaced until 2023.
In 2022 we will get the Mac line-up finished and a start on replacing the low consumer end of the line-up.
If you need/want a MBP now is the best time.
I think you may be right, and even if there is a small update next year, it would probably be to include some of the core improvements from the A15 design into the M1 Pro and Max. We don't yet really know what Apple's 1-2 year cadence will be for A-series, M-series and M-Pro/Max series, and the current order, i.e. A15 iPhone/iPad before A14-based M1 Pro/Max may not apply in the future. Or it may; we don't know!
In any case, I doubt we will see the huge jumps that we saw from "entry-level Intel -> M1" and M1 to M1 Pro/Max again...at least not with the current lineup. Even the jump to 3nm Silicon is unlikely to shift the performance dramatically - it might be a 30-50% jump for some metrics.
Getting on the train at the first station gives you a longer journey to maximize the benefits on an upgrade 3-5 years down the track. I made the "mistake" (unknowingly) of buying an upgraded i9 MBP16 at one of the last "stations" before the launch of Apple Silicon which means I'll be upgrading much sooner that I planned for.