I agreed with your great point.
There are some indicators for good forums:
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History (its origin, and its original purposes): In this indicator, there is no forum better than bitcointalk.org forum, definitely.
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Members: especially active members. Only figures on active ones should be counted, inactive ones should be excluded to avoid bias.
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Contents: it is obviously important, because no one have interest to join forums in which tons of trashes spread around.
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Existence of top old, well-known projects: Such as bitcoin, ethereum, dash, monero, bitshares, clams, etc. In this indicator, once again bitcointalk.org forum is the best. There is no competitor in the indicator, by now.
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Strict rules, and good forum managements: In my opinion, the forum has still been the best. This is why forum users tend to spammed complaints when some big changes made, but still kept joining the forum, not leaving it.
Guys, there are no need to compare these forums with Bitcointalk. Simply it can't be compared by history, number of members, content and etc. But some of these forums can be partial alternative to Bitcointalk, some are for people who interested in specific altcoin. There is also some good non-English crypto forums like Bitalk.org
It's the point, too.
So, in summary, none of the above come even close to Bitcointalk neither in terms of content nor, very likely, prestige and history of its members.