What's so good about No 802.11ax Wi-Fi?
Would mean I need to upgrade my wifi router which I haven't long bought (and it cost over £200! haha)
In an uncongested home network, not too much. It may bring higher throughput but it will depend how it's deployed. Aside from the 1.4Ghz 13" the current Pro range all scale up to 1.3Gb/s (using an 80Mhz channel), which is far more than most people have at home anyway. The 1.4GHz 13" and the Air range are limited to 867Mb/s.
Most poeple won't need either 1.3Gb/s or 867Mb/s, but the effect of 3x3 vs 2x2 antenna deployment means that as you move to the outer reaches of your network range you're still supporting higher speed. So 1.3Gb/s vs 867Mb/s may not matter too much, but at a larger range the difference may become more noticeable. And if you're using NAS for Time Machine then you have 50% greater bandwidth.
For ax, the key benefits are more efficient use of the available bandwidth for bursty traffic flows. So when you have 20 devices attached the performance degradation will be FAR less on 802.11ax than on 802.11ac, with 802.11ax bringing some "full duplex"-like characteristics. Higher through will be available too, assuming it's deployed that way.
Although there are 802.11ax laptops around already, I haven't seen any deployed in 3x3 fashion, only 2x2. If the MacBook Pro with 802.11ax is 2x2, then the theoretical throughput vs the current 3x3 802.11ax deployment is 2.4Gb/s vs 1.3Gb/s. That's assuming 160Mhz on 802.11ax vs 80Mhz on 802.11ac. If we're somehow stuck with 80Mhz 2x2 on the new 802.11ax, then it's 1.2Gb/s vs 1.3Gb/s, so worse off during large max bandwidth transfers, but you'd still get the benefits for your smaller bursty traffic flows. But then if you're in a crowded neighbourhood (from a wifi perspective) you may not be able to hit 160Mhz anyway, since that's basically taking all the "air space" and saying "Mine".
Ultimately 802.11ax may provide a huge uplift in the right situations. If it's a half hearted 2x2 deployment then it's probably no better than 3x3 802.11ac for the average user.
Yes, rip and replace your router if your router doesn't support 802.11ax. But you would be able to use an 802.11ax Macbook with an 802.11ac router, running 802.11ac, not 802.11ax.