It's not really about validation. Either MyServiceClass works or it doesn't (whether that's DX, or some other problem), and if it doesn't you make a new one or fix it. Most code isn't so complicated that coming up with the correct method signature is prohibitively difficult without writing tests.
If down the line the way it was written isn't working, maybe you need a V2. Anything important that needs updating to V2 gets updated, anything not worth the effort stays on V1. Usually that's not an issue. Sometimes you have to drop V1 for one reason or another, and that can result in some work, but I reckon it's time saved vs. writing test suites, and it always feels worthwhile to do, vs. writing insurance code.
> Without tests, code bases just become increasingly haunted graveyards where nobody is willing to change anything.
People will be fine with changing things if the cultural standard is that bugs can happen but just try to avoid them. As long as you have good logging it's largely a non-issue. I'd be more hesitant to change code if I had to update 10 tests along the way. Maybe that's laziness but it also feels wildly unproductive.