>But a "Full" Steem account cost 3 Steem or RC.

*OR RCs*

There is the potential to put both accounts and RCs into pools for usage. There is also the potential for guest accounts to be used in multiple ways so only those currently logged in get a seat, and it drops away once they logout and it gets suffled to the next user. This would work for games. 

>And If a Forum with only 5k Users want to onboard without guest accounts ( Because there want to use there own SMT), it would take forever.

There are several ways around this, and SMTs do not necessarily need every user to have an account I believe as a website can integrate it into their experience and have the blockchain track the transactions for individual users to follow their earnings/rewards/or whatever they set the SMT to do. They don't all have to work like Steem works now and I think that once in the wild, developers can get quite creative and generate many transaction (freely) without necessarily needing many user-held wallets. 

Not my area though.