The venn diagram of people who are capable of operating a secure Linux or *BSD based email server implementing, for example, SPF, DKIM and DMARC with postfix+opendkim+spamassassin+dovecot overlaps a great deal with the sort of people who want to colocate a $100-200/month server. A lot even have "free" colocation through their work at an ISP or with friends that have extra rack space and power for a small 1U box.
It certainly doesn't make sense to spend $200/mo on colocating a server just for email - but if you're colocating a whole physical server in this era, it's not hard to make it a box with 64GB or more of RAM and two good quality 512GB SSDs in RAID-1: Make it a hypervisor platform and put twenty of your own VPS on it doing many different things. Balanced with the need to not centralized too much stuff on one piece of hardware as a single point of failure. Public IP space availability depending, of course.
In my experience the best colo is with ISPs that are not actually colo companies, but will only do it as a side thing for people they know and trust. The absolute best colo I've ever had is with a company that has a core business doing X.509/SSL stuff for healthcare enterprise customers. Gear in racks two hops network-topology away from their core routers at a major IX point.
The vast majority of people who do not want to maintain and secure a world-facing Linux/BSD based smtpd are probably better off going with a google apps or hosted email solution where all of the smtp and imap/TLS1.2 infrastructure is handled for them.