I think that many of the "normal" cabin crew are not particularly interested in or motivated by the BASSA leadership. I think that many of the "normal" crew see the value of being in a trade union that is the formally recognised negotiating body with BA. BASSA have over the years created an enviable set of T&Cs for their members.
Many crew believe that this will be sorted and when it is, they will still need a union to protect them in the small things, a place to go for guidance and assistance if they have a legitimate dispute with the company, say over absence or some such. BASSA, like most unions is not all bad. The leadership are way off base in this current dispute but that in itself does not negate the positive benefits of being in a TU.
There is no sensible alternative for CC, the PCCC is a toothless and faceless thing at the moment and can do nothing to materially assist any potential member as BA does not recognise them.
BASSA may well need reform and I think that it's constitution is deeply flawed - a show of hands can postpone elections. Even during the WWII we had elections and Churchill was thrown out - so there is no democratic reason to not hold elections. DH is holding onto power and influence undemocratically but that is a matter for the future.
Leaving BASSA right now may well be throwing the baby out with the bathwater, leaving crew exposed in industrial matters but also allowing the union to become ever more extreme. Others though, are just apathetic.