Thanks for taking the time to read and respond Alejandro! I think it hasn't made sense to host your own servers... maybe ever, but definitely not in the last 20 years, and especially not today.
Even in a server farm, I wouldn't be surprised if few places in London have adequate air conditioning for those 100 degree days (40+ degrees celsius)...
"Let’s get to the important question, do you need air-conditioning in London? No, you don’t, unless London is experiencing a heatwave. The temperature during summer is an enjoyable mid 20’s (70°F) during the day and mid 10’s (55°F) in the evening. Most British housing will not have air-conditioning and will opt for heating instead." source: https://katsgoneglobal.com/do-you-need-air-conditioning-in-london/
All that being said, I think the mentality was "Since we're no longer going to be managing our own servers anyway, is MongoDB still the right choice for us?"
I would have landed on that being a separate project: 1) move the servers, and 2) consider a transition (or consider waiting for features), but their engineering team tackled it all at once. As far as I know they're happy with Postgres!