I'm not disputing that assertion, yet it does go against the marketing materials we're all presented by all of these services, as for reasons to not run our own mailservers.

In other words, if all you want to do is run a personal mailserver, or even a corporate one, you'll probably not have to deal with this supposed IP reputation issue, unless the IP addresses you use, have already been added to the blacklists even before you start at it.

Running your own mail servers to do the volume emitted by Sendgrid would indeed be on the level of starting your own medium sized business. Getting IP allocation, swip'ing them out to divisions of your company or your customers and paying into whitelists for all the "free" email providers like Google et al would be a massive up front cost.

Running your own mail server for personal email is an afternoon of setup DKIM, DMARC, SPF, FCrDNS and such, setup of your MTA/IMAP/WEB preferences, tuning some filters, setting up aliases, accounts for family and with time the tuning work eventually slows down and then it's just maintaining accounts, aliases and the occasional rules to block problem networks and domains. With time you may find some servers that require lowering security or filters but that is also very easy.

Yes if you are using a domain that's been around for a while and has a reasonably stable IP address history and is not on any blacklists, that is the defintion of a "good" reputation. Or at least it's not a bad one.