Most of the Bears fans I know have moved on, and of the ones that are still around a lot wouldn't support the Bears unless they were the NS Bears or the CC Bears...
I have no doubt that if the Bears were to come back they'd have huge support from the old fans in Sydney for the first few years, but almost all of that support would drop off after the initial excitement had waned and almost all would fall back into the normal routine of their lives and wouldn't be ardent supporters of the club going forward.
Honestly I don't really have an opinion on a Perth Bears bid, I think it's unnecessary for Perth and only really benefits the Bears, but I'm not against the idea either, but the amount of actual active support that the Bears could draw these days is massively overstated, and frankly the talks of a huge support base of Bears fans waiting to re-emerge is beyond fanciful.
Realistically once the initial excitement had died down the Bears would be lucky to draw a few grand in active supporters from the old Bears fan base.