Yes, there are lots of fans of the legacy fleet here (the amount of usernames beginning with 507/508 just goes to show, haha) and thus I notice a bit of bias - the 777 faults are always moaned about, but the 507s have been problematic themselves of late!
The thing is the 507/8s have been a bit neglected for years - I certainly noticed commuting on them in 2020 that they'd started to get the "don't spend money on them as they're going soon" treatment, I was expecting them to be woefully unreliable by now but they just keep going. Some of them have done 46 years of service and if one breaks down its sort of expected. The 777s were rushed into service fully knowing they weren't ready (according to a friend of mine at Stadler) due to pressures by Mr Mayor so he could have his 5 minutes of fame, a photo in the paper, then vanish when things go wrong. Teething issues are of course expected with any new fleet but they have been suffering consistent faults since day 1, and rather than halt the rollout for a bit and get things sorted they've thrown them out everywhere and as a result decimated the network reliability - arguably what makes it so successful in the first place. The Kirkby line being a fine example, they used to have a clockwork 15min frequency which was very reliable. They've had frequencies slashed for the best part of a year, many turning to the car or bus, then when the train does run it breaks down..
I work out of Chester station, I am (was) a regular user of Merseyrail and have been for the past near 30 years - these past few months since 777 introduction have been dreadful. I've never seen so many services cancelled on the line in my life. As a result it is too unreliable for me to use as a commute anymore, so I'm now commuting by car. I sincerely hope things improve, but things should have been done very differently.
Presumably due to 508108 & 508114 not leaving the network on Monday and the unit shortage of the 507/08 fleet at present?
Absolutely bonkers with the current 50X shortage and chronic unreliability of 777s (even a number of 777 services on Wirral cancelled last week due to shortage of 777 trained staff), that they withdrew any of the last few 508s at all (508103, 108, 114, 120, 131, 136, 141) as all of those units were not long overhauled and had years worth of miles left - there are 507s running around much closer to overhaul time and in worse nick than some of those 508s too.