The real answer is to cultivate a team that can work on the project long term using the old tech for significant gain. Your turnover just needs to be slow enough that new developers can be ramped up properly.
I did a gig at a place that brought me (along with like 50 other people) in to transition from a server rendered PHP app to a React front-end. They'd bootstrapped to something like $300M and had just brought in a new CEO who wanted to 10x the company.
It was fucking pandemonium. Every day I saw another message on their slack along the lines of "It's my last day, I had a lot of fun working with you all the last 8 years". My team was completely useless, as far as I could tell our mandate was to basically be nazis and tell all the supposed mouthbreathing pre-historic over the hill PHP greybeards how to write "good" React code (aka unmaintainable garbage).
I have no idea how they're doing now. I don't see them mentioned much anymore. They're probably worth $30M.
If you had to bet on who could 10x your company, would you go with the guys on the ground floor that have literally bootstrapped a company to $300M off the back of their PHP tech? Or would you go with the army of hip Javascript consultants, half of them off shore?
I say this as a specialised React developer, who has been doing solely React for 2015.
There's absolutely no reason why you have to roll with the industry to every new fad. I have no intention of going anywhere when the next thing comes along. In fact, I'm waiting for all the noobs to fuck the hell off so I'm free to actually write good React code again. I bet that's probably how the PHP developers felt back when that was the fad of the decade.
If that company had backed their PHP developers in and given them enough shares to stick around, I bet they'd probably be a $3B company today. 90% of that is going to come down to the business side of things, not whether you throw away your entire engineering culture to score 20% better on some fucking render metric on a page transition or whatever.