for a long time the forum only accepted btc because, well, bitcointalk.org i guess. altcoins were specifically excluded iirc.
grin was the sole exception due to its tech i believe.
It's best if it remains restricted to Bitcoin or Bitcoin side chain and off chain scalability solutions.
Theymos cannot guarantee if an altcoin project will remain the way it is 10-20 years down the road just as Bitcoin has been. If the forum adopts another payment option, it's an endorsement of it, a huge one at that. If it fails it gives the forum a very bad look and leave a gap open needing to be filled. Could easily just be one failed experiment after the other.
The evil units fee should have already been discontinued and a LN Chanel opened for copper membership payment.