Well, that's the problem, stated also in the OP - you have to wait. You reach already your merits (if you are a good contributor) and you wait. If more merits were required, you would be incentivized to contribute more and more, instead of just waiting.
Anyone who is able to consistently earn more merits than activity over a prolonged period of time will be, as you say, a good contributor. Users such as these are unlikely to be here simply to spam for some signature or bounty campaign, and are instead here due to a genuine interest in bitcoin. They are unlikely to stop posting and just wait due to some arbitrary activity number.
Not including theymos and satoshi, there are only 83 members who have earned 1000 merit ever, in the >2 years since merit was introduced. That's not even 1 new legendary a week. I don't think that's an unreasonable position. Increasing the limit to 3,000 would give a grand total of 5 legendary members on the entire forum (again, not including theymos). I don't see the benefit of doing that. The merit system was supposed to prevent spammers from ranking up, not create a new rank which is only obtainable by <0.001% of the forum.
I would agree, however, with increasing the threshold from Junior member from 1 merit to 10 merit. It is still too easy for spammers and account farmers to beg, buy, trade, use alts, or plagiarize their way to single merit and then start signature spamming for some meaningless altcoin. I'd also agree with removing airdropped merit for the same reason.