I don't know if a cave in would have worked, but given the location where they were, it would have been impractical. Making major "readjustments" to the welcome center in general is a "bad idea". The two major possible results would have been to either isolate the fortress from the map edge so that caravans (at least those on foot and not on wagons) couldn't get to my fortress (this would include the liaison). Or the effect would have been to grant access to the fortress of unfriendly creatures such as ambushes, or sieges. The way that I play, by the end of the 1st year, I have complete unfettered access to the surface of the embark, and only flying creatures are capable of getting anywhere other than the 3 or 4 tiles next to the map edge unless they go through the welcome center. So sieges are a mere inconvenience while they grind themselves to death on the welcome center. Traders just come wandering in, trade, and leave. In fact, the only ground bound "hostile" that's capable of reaching my fortress are Kobolds. But they get discovered before they get to anything valuable, so they're just a nuisance. And now with a fairly large number of crossbow armed dwaves, those kobolds are a rather short lived nuisance. Kinda of like the stock scene of a robber coming into a bar and attempting to hold it up, only to find out that the bar is a favorite hangout of the police....
And just in case that I get a surface hostile that can fly, I do have the capability to pull a lever and close off the entrance of the fortress entirely.