They are easy to level 'for now'. They can go up to level 90 in 6.0 so the system would be grandfathered in, same for any other expansions.
A lot of people wouldn't like joining because they're missing out on the EXP.
There's other things that can be said, but I don't think this matters.
Plus with you saying 'just being able to play with people who has issues with grouping in big parties, and want to go at a slower pace' you're literally just saying what a party with trusts do.