In the short term, I agree. But if you remember the conversation with Lord Dace in the Dwarf Noble Origin giving the castless rights has an economic impact on the noble houses. I'm not convinced Bhelen was interested in helping the castless. I think he was interested in weakening the noble houses to make his position stronger.
Dace wasn't looking to give casteless rights, he wanted to restore the castes/Houses of surface dwarves. This would have cost the other Houses (namely Helmi and Aeducan) money because a surface guild containing formerly Helmi and Aeducan - among other Houses - dwarves owed Dace a lot of money and restoring their castes would obligate the noble Houses to pay up. '
One a side note, how do you get to be heir as a Dwarf Noble? I played one through once and didn't see anything in the epilogue.
The dwarven epilogues say nothing about who succeeds either Harrowmont OR Bhelen but there is a good chance that should a competent DN be in Orzammar and interested (they don't have to fail at politics just because Bhelen is better, after all) that they can succeed Harrowmont. In addition to all the other advantages (heir to House Aeducan, Paragon, ended Blight, common people love them, enemies in Assembly dead in coup), Harrowmont is also fond of them so wouldn't have a problem with them taking over.