Honestly, I think your friend is right, it’s a question of economy of scale. As you scale up there will be less and less wasted resources in overhead. Once you reach the scale where you need hundreds or thousands (or hundreds of thousands) of servers to operate your site you’d likely be able to fairly efficiently dimension the amount of servers you have so that each server is pretty efficiently utilized. Youd only need to keep enough spare capacity to handle traffic bursts, which would also become smaller compared to the baseline load the larger your site becomes.
Realistically most self-hosted setups will be mostly idle in terms of CPU capacity needed, with bursts as soon as the few users accesses the services.
As for datacenters using optimized machines there is probably some truth to it. Looking at server CPUs they usually constrain power to each core to add more cores to the CPU. Compared to consumer CPUs where at least high-end CPUs crank the power to get the most single-core performance. This depends heavily on what kind of hardware you are self-hosting on though. If you are using a raspberry-pi your of course going to be in favor, same is probably true for miniPCs. However if you’re using your old gaming computer with an older high-end CPU, your power efficiency is very likely sub-optimal.
As a “fun” fact/anecdote, I recently calculated that my home server which pulls ~160W comes out as 115kWh in a month. This is a bit closer than I would like to the 150-200 kWh I spend on charging my plug-in hybrid each month… To be fair though I had not invested much in power efficiency of this computer, running the old gaming computer approach and a lot of HDDs.
That said there is plenty of other advantages with self-hosting, but I’m not sure the environmental angle works out as better overall.
I’ve set up a https://stalw.art/ server recently, and I’m quite impressed. I appreciate that the entire mail service stack is taken care of by a single unified service, except webmail but Bulwark seems really solid. It also works nicely together with Postgres and S3, so I can have the same backup strategy as most other apps I’m hosting.
First and foremost for the outgoing needs of https://nord.pub/, but I’m seriously considering moving personal domains to it as well.
For infrastructure I’m using dedicated Hetzner hosts, with extra IPs for the mail servers, so that reverse DNS is consistent.
Largest problem I’ve seen is that Outlook.com is classifying the emails as spam, even with SOF, DKIM, DMARC properly set up… which is a big reason I’m hesitant to move all personal email as well. I realised that it could be a problem if I ever want to contact companies who use Office 365, which is a lot.