Unless you have an obvious and accessible way of getting secure third party builds whitelisted, this is still a very anti-user approach, which is not justifiable unless the user of the device isn't its owner (like with company-owned work phones).
Keep in mind that Pinephones and similar are a thing. Lots of people are hoping they don't fizzle out and die off like previous "open" phone projects. :)
If a company wants control over devices they own, that's still fine.