Depends on who you talk to and the type of variety and balance you could provide with an alternative diet. To me, it makes more sense to offer a diverse and nutritious fresh diet to supplement a variety of hays. But some buns need the pellets (even a tiny bit) because it’s not fiesabe to offer enough fresh food variety. Also, bun tolerance is a factor as well. So… no clear answer I’m afraid.

Some pellets can offer vitamin D3 (which can be lacking in house bunnies since they may not be exposed to direct sunlight). I agree with Q8bunny, it really is who you talk to. Wish there was one simple answer to your question.   I have had to have a bunny on a no pellet diet before, (due to high calcium sensitivity), and he did great — but he also had access to sunlight through a window lounge area. How are they doing now with the current probiotic pellet? What is the nutrition difference between that and the pellet you are considering giving?

Ive met some rabbits that have been on a hay/veg diet for a number of years and they are very healthy. One friend only just started with occassional pellets again as her rabbits are getting elderly.
Maybe you could give the buns something like Oxbow’s multivitamin supplements as an occasional treat as alternative to regular pellets?

I’ve thought about going pellet free a few times. Mine only get a tiny portion each day, so they see their pellets as more of a treat food than a main part of their diet. But yesterday, Jenny started going into stasis. She does this every so often, but thankfully responds well to gas drops and tum rubs. After, she is always ravenous for pellets, it has to be pellets, she maintains, and I wouldn’t feel comfy let her eat the amount she does if they were no longer a food she was used to. So I go with a tiny portion each day, just so that pellets can be used in great quantity when a bun is poorly.