But is training + market rate more expensive than hiring + market rate ?

Also, it seems to ignore the fact that training people is actually an incredibly useful thing to do to hone your skills as a senior dev, and that having to teach forces you to cristallize, simplify and explain thoughts and processes that you possibly never challenged before.

Tbh, I think I'm more productive and learning more when I have a decent intern to coach in my team than otherwise. So it's really a win-win situation.

While this is true, it seems to me that most companies don't actually have both choices. Instead it's "training + market rate compensation" or "you will forever struggle to have enough senior devs to accomplish your goals". If you can't attract talent, you have to grow it and pay to retain it. Simple as that, no?

Even if your junior-turned-okish developer goes away, if you did a good job and part on good terms you now have a working advertisement in another company. And maybe a couple years down the line they'll come back.

Investing in your workforce for the long term: when you're big enough I think you should do it. Having a "reserve" is also useful. You don't have devs doing nothing: you have devs learning, teaching and ready in case a business opportunity appears.

I think you also have to account for the cost of acquiring senior staff. If you train them and pay them, your pipeline becomes more predictable than plucking them ad-hoc from other companies. That makes planning (your roadmap) more predictable.

Market rate for junior + training while he/she is junior is not more expensive.