As far as I understand the situation it's not like the 'full expierience' for this one game includes 2000 songs though. These songs have accumulated over the course of at least three full priced retail games with 54-80 songs each and I don't know how many expansions.
While i do think it's a problem that they disabled the import for the earlier Rockband games into Rockband 3, you could theoretically get all of those ca. 200 - 500 songs at no additional cost for Rockband 4, because you have payed for them before.
I still think the songs themselves are probably a bit overpriced, but I also don't know how pricey the bundles are.
It would be cool If they had a legacy bundle that includes all the on disk songs from Rockband 1-3 for a reduced price though.