Also keep in mind that it may be slower than with direct in person mentoring but there can also be the opposite of in person mentoring and then being locked in the same office can become extremely stressful.
I also run a small team with new hires on one day and I'm seeing great productivity and teamwork.
We are going back to the office eventually but I don't think the on-site requirements will be as strict as in the past. In the medium term I see us moving towards a more flexible model beyond just the flexible allocation of desks (which imho. is a horrible thing, pretending to be office workspace and serving neither then needs of people there nor the companies output).
I would probably refuse some a new position if there was the expectation of constant pair programming, but I'm curious if others have different experiences.
My employer has always (started in 2007) been fully remote; we've never had office space ANYWHERE. It's a small team building a relatively niche product, and we're pretty successful, but what you say is true:
Getting new people up to speed is hard. And hiring "baby developers" is just not something we've done. All our hires are midcareer or later. I feel like a fresh dev needs the office environment to get traction early in their career, and we just can't provide that.
I assume it's a bit easier if you are more senior and have a few company changes on your resume, but for someone for whom its their first or second job in the industry, it can be overwhelming. I think that's why often fully remote companies pre-pandemic seemed to be more focussed on hiring seniors.
Likely some of these problems can be mitigated as we learn how to deal with them, but this is one reason why I think post-pandemic most software houses will have to adopt a policy of encouraging some on-site work.