When Clowney signs the franchise tender, his contract can be transferred to another club (prior to the trade deadline). But the stipulations remain.
Clowney could sign an offer sheet from another club. And the Texans could match it, or accept 2 1st round picks from that team in compensation. Do you think that's happening? It hasn't happened yet. That's the only way Clowney can get a multiyear deal this year.
I think they can negotiate those 2 first round picks down. But yes that's the big obstacle. Texans will probably want at least one first rounder and that makes acquiring clowney more expensive than just giving the extension when he hits free agency