They always play it super safely. Many other prog bands will play their "classics" realizing that this is what many fans want to hear.
I would venture to guess that in 2021, at least half of the audience in any of these Genesis strongholds that they'll be playing in would want to hear some representation of '70s stuff, not just '80s pop. Many of the fair-weather fans have long left the band (the bandwagon jumpers in the '80s/early '90s).
You're far from alone in what you expressed. But I'd say re the above that the other prog bands you refer to are the ones who are playing it safe. I always enjoyed hearing 70s stuff in Genesis sets but accepted they were a band who emphasised the later material. I don't think it was disregard for their 70s stuff as you put it, more that they were mainly enthused by playing their recent work. While I liked hearing older stuff I'll always respect a band who want to shine the light on their new stuff. Bands whose live work rests heavily on older glories - I have less respect for them but if it's what they prefer to do, and what fans prefer to hear, I get that. But I think it's a pity for any band still actively recording and releasing new stuff to pander to fans who'll just moan about it. Fans should expect a band to play a lot of new stuff and if they don't like it, don't go. Whether a band doesn't play their new stuff because they themselves don't like it, well that's a whole other matter they need to seriously address!
I don't like this characterisation of fair-weather fans. It'll be the same for any band that fans who came on board 30-40 years ago will possibly have drifted away, possibly to other musical pastures. Pretty normal and absolutely nothing wrong with that. With Genesis there are fans who grumpily say they abandoned them after Hackett or Gabriel left, or after Duke. Are they 'fair weather fans'? You need to be careful about labelling some fans as bandwagon-jumpers, you could be dismissing many of the board members here, who discovered the band through IT or WCD.