The issue is more that some AX features (OFDMA UL, TWT) might not get enabled in older, first-to-market routers. That's unlikely to cause problems for most users.
Devil's advocate mode again..
Except that TWT, UL/DL OFDMA, and UL MU-MIMO
are the keystone features of 11ax over 11ac Wave2...
Once one rules those out - physics takes over, and noise becomes the limiting factor again - one can only push so many bits into the stream as per Shannon.
Most client STA's are going to be 2*2:2 based, even with 11ax, so outside of someone copying large files (seriously, how many people do that all of the time???) one will really be pressed hard to tell the difference between an AP that is WiFi 6 vs one that is WiFi 5.
The 802.11ac Wave1 feature set was the key advancement in 5 GHz... nothing else has come even remotely close to Wave1 - Wave2 and 11ax are incremental improvements at best
Don't get me started on the marketeering of 6E, where the only benefit is to the chipset vendors and OEM's by doubling the number of radios sold - 6E will need to have a dedicated 6E radio forever, unless one banishes non-6E clients to the ghetto of 2.4Ghz - which obviously won't happen as consumers don't want to move backwards...
2.4GHz, not much benefit there either, as 802.11n client STA's will be the norm for many years... some chipset Vendors (MediaTek) focus their solutions outside of 2.4, reducing cost by using either 2 stream or 4 stream WiFi4 solutions. The best thing that can help 2.4GHz is to remove support for legacy modes for 11b (DSSS/CCK) and force folks to use OFDM there (11g/n) - airtime is precious, and getting rid of 11b legacy support helps reduce overhead by at least 50 percent, and even more when considering that by removing legacy in 2.4, one does not have to do multicast frames at non-OFDM rates, and we can remove the "legacy" protection modes (ERP use protection by sending 11b CTS-to-self for example).
This is essentially free, and can be done on most AP's with a software change... and on many, it's not even that, it's a config item, so it's essentially free - which doesn't appeal to the OEM's and Vendors, because then we don't need to sell more chipsets.
Getting rid of legacy support that will obviously help everyone in 2.4GHz, and this can be done solely on the AP side without changing clients, and this is actually more of an improvement than pulling in 11ax into the 2.4GHz band without having to replace client stations.
Let's be really honest about WiFi6...
Most of the real world "benefit" of 11ax is confirmation bias, where "it must be better" because one spent $500US on a router AP that has 6E and all the gubbins, except that again, the keystone features (OFMDA UL/DL, MU-MIMO UL/DL, and TWT) aren't actually implemented, or implemented badly (OFMDA-UL on Broadcom vs. QCA for example).
Again, just being the devils advocate here, not trying to be combative.