I went through this recently when I joined a company and a small team to do a particular overhauling project. One of the developers was relatively junior, but smart, driven, and in love with shiny new tech. For the particular piece he worked on he chose modern solutions that went a bit out of the companywide envelope. I figured it's fine since if we forced the "boring" company-standard set of technologies he might get bored and leave. Well guess what, he left anyway and now we're left holding the bag. All custom stuff that could have easily been reused: scheduler, base container, code generation, language. So I guess my big takeaway is, regarding retention, they may leave anyway. If you're going to open the "new tech faucet" to slow that down, keeping it sequestered may be worthwhile.

I've seen an internal tool at a previous company that seemed to be built similarly by developers who wanted to try new tech. A pretty fancy RoR with a lot of custom changes and integrations. I am sure they had a ton of fun building it, but again, guess what, they were all already gone by the time I started there. And those who stayed hated that code base with a passion because it was near impossible to upgrade, so now we were stuck on an ancient RoR version, and since it was integrated with a lot of other systems it blocked those upgrades as well. Cautionary tale I guess.