This is great, but only when you've already got enough knowledge and experience within the organisation to enable it. That's where you really need to hire senior developers. Trying to grow a team of juniors into a team of seniors without enough guidance, grows teams of expert beginners, which is way worse for both the team and the organisation in the long run. You won't be able to hire or retain seniors in that sort of environment as they'll see it for what it is and leave.
However, that's not to say training people up shouldn't be done! I've seen some wonderful developers grow from junior to senior and it's always great to see it happen. Just make sure the organisation is in a position to do it well, otherwise you're doing both them and the juniors a disservice.
That's a good point. One of the places I worked hired a ton of juniors and placed them under "senior" team leads, but the leads were too few, not exactly all-stars, and code quality was all over the place. I tried to have a positive influence on some of the eager new hires, with varying success (mostly because I wasn't very senior either, just a little further along than they were).
Eventually the environment became frustrating enough that I had to move on. Great people, really friendly environment, and even a product I believed in, but it felt so bad to keep having to read through awful, giant, bug-ridden code reviews with the same old rookie mistakes. I only had enough influence within my own team to slow the rate of bad code entering master, but it still came alarmingly fast. I think nobody from my team is still at that company. I really hope management has turned it around since.
However, that's not to say training people up shouldn't be done! I've seen some wonderful developers grow from junior to senior and it's always great to see it happen. Just make sure the organisation is in a position to do it well, otherwise you're doing both them and the juniors a disservice.