Eh...this is pretty much bullshit. Unless you're running your own, good-reputation AS (or happen to know someone running a clean AS who can either lend you an IP or announce a clean block for you), the /25|/29 assigned to you by your colo provider has just as good a chance as being dirty as that VPS providers.
Your colo provider is also more than likely happily delegating blocks to one of those cheap VPS providers out of the same larger netblock your IP. Chances are even good that you'll be given a /26 or a /29 that used to be used by one of the VPS providers.
If you see a provider offering "up to 256 clean IPs!!!", those aren't clean IPs. Those are, at best, greylisted IPs bought on the cheap and at worst /22's rented from poor rep /16's and rebadged in the hope no one will notice
On the customer side, one of the major bars to entry for clueless/spamming customers is whether it's possible to directly purchase hosting services online with a credit card and have them immediately provisioned and available. If you can get a VPS in 5 minutes by paypal it's easy to be clueless. If you need to set up and ship a server with its rails to a colo it's likely but not guaranteed that you have more clue than usual.
Yes, it's likely you might get an IP in a /26 that's part of a hosting company's much larger /22 or /20 which also contains shitty VPS. The key part there is to find a hosting company/ISP that doesn't do low budget hosting and never has.
The number of colo providers who refuse to allow customers like VPS hosting companies (who buy rooms not just rent a piddly little 2U from a shared cabinet), is pretty small.
All in, your advice isn't tenable for the majority of technophiles, let alone your slightly-more-technical-than-average user who wants to setup a mailserver. The barrier to entry, your way, is insurmountable.
Maybe it's time for a startup in the "DIY mailserver" space, that offers clean IPs for personal use mailservers and hand-holds through the process?
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.
You've just cut the number of people who can do it your way down to <1000 people (or may as well). No normal person, who wants to learn how to setup a mailserver and be successful at it, is going to drop ~$5k on a server or know people who run T1/T2 datacenters and are willing to pop you in a cage for free.
Your last sentences dismisses everyone who doesn't do things to your impossible standards as a waste.
You're basically saying no one should run a mailserver, or learn how, or be given the opportunity to learn how. Which is sad.
There's very little that can be easily done about this other than moving your smtpd to an ipv4 address with an ISP that has never had an outgoing spam problem (such as for example a /24 that's been held by the same company for 8+ years, in ARIN/RIPE/APNIC/whatever space very tightly controlled by the network engineering team of a clueful local ISP where you know the staff).
There's a pretty direct inverse correlation between the cost of a hosting service ($5/mo VPS vs. minimum $200/mo colocation of a 1U server) and how much outgoing abuse traffic has been sent from the particular netblock assigned to the enduser customers. Cheap hosting company = poor IP space reputation.