Let me try to make my argument again now that I've thought out a better way to say it- pellets have enough fiber and animals can last quite a while on pellets (particularly if their genetics are in support of longevity) but obesity (most rabbits- not all- fed only pellets are overweight if not obese) does drastically reduce lifespan. So assuming an owner is careful about obesity, pellets are not usually a problem (except in those animals that have conformation issues with their teeth- they tend to need the extra grinding action required of hay). Rabbits prone to ileus also seem to do much better when given mostly hay and the number of pellets reduced, in my experience working at hospitals, as well as those prone to cecal dysbioses. A normal, healthy rabbit would probably do just fine on pellets alone, like Ladysown said. I personally would not feed my rabbits only pellets if it can be avoided (some rabbits of course just will not eat it)- I tend to feed only hay and veggies, with pellets more as a treat. But that's my personal take on rabbit husbandry.