Better? Really? Care to name several "prototype" 4-3 DEs who suddenly got "better" in a 3-4? Bruce Smith was told that he'd likely have had 100+ more sacks in his career in a 4-3. I like Mario, but he's no Bruce Smith.
Agreed, but a vet at 3-4 DE is a completely different spot than what Mario has played thus far.
The market would likely never be higher for Mario than it is right now. Those of us who worried about going to a 3-4 knew this all up front. Suppose that Mario isn't willing to put in the extra work required of a 3-4 DE? His value will plummet if his production drops like a rock...which is entirely possible.
I'd keep him forever in a 4-3, but this is literally a make/break season for the Texans and he's never shown the abilities that you generally look for in a 3-4 DE. I wish him all the luck in the world if we keep him, but look for his production (and happiness) to fall.