Your 802.11ac clients will benefit from sitting in an 801.11ax infrastructure, as long as there are other wireless devices running 802.11ax.
Certainly it would be great if it had Wifi 6, but Apple did a particularly strong wifi deployment on the MBPros (aside from the 13" 1.4) and so the gap to Wifi 6 is not as great as it might be with some other vendors. If you're going from a 2019 1.4 13" to a 2020 13" with 802.11ax the gap is larger. But even then, the 2019 1.4 13" will still benefit from being in an 802.11ax network, even without running 802.11ax itself, as long as other devices are running 802.11ax (iPhone, iPad etc.).
Wifi 6 is high up on my list because I use a NAS at home for local storage, so Wifi is a key component in my overall workflow velocity, but at the same time my Internet access is only 40Mb/s, so when I'm surfing the net Wifi 6 really brings little advantage.
If your main usage is surfing the net and if you're not running a Gigabit link to the Internet the benefits of Wifi 6 are not huge. Nice to have, but not huge.