Do you have evidence for this? Intro points are used temporarily to establish a connection, but the bulk of the bandwidth is distributed through the rendezvous points. That's one of the reasons to use rendezvous points. That seems far fetched. Take this forum as an example. There are 100-150 concurrent users, but how many are accessing the forum for the first time at any moment? Probably not more than one every 10 seconds. 6 new users per minute, 60 in 10 minutes. That seems like a fair estimate. I'm sure three intro points can handle 6 or even 20 users per minute. Even if the main site has 3 times the traffic, it doesn't seem like they would get DDOSed. I don't think that's going to be more than a few hundred people, even on the busiest hidden service.