I don't understand your assumption that a tampa-2/cover-2 scheme is incompatible with and necessitates a move away from a 3-4 DL.
It has to do with the linebackers and the physical attributes you need for each system. The 3-4 scheme requires the outside linebackers to be very big guys, 240, 250, 260 pound guys who have to be able to set the edge against the run as well as pass rush against prepared left tackles. Their pass coverage responsibilities are the flat zones, and short drop zones. The tight end is usually the responsibility of one of the inside linebackers [ this is what Roman Phifer did so well for so long.] The OLBs are often fast and slightly small 4-3 defensive ends.
The inside linebackers for the 3-4 are the generalists; they need to be big enough to take on guards, and at least one of them needs to be able to keep up with a tight end down the middle. However you'll almost never see an ILB with zone responsibilities past 15 yards deep --- you'll see one running to keep up with a seam and fly routes, but that is not their primary responsbility.
Now the 4-3 Tampa-2 has totally different requirements for their linebackers. The OLB is responsible for the tight end and the intermediate zones[5-12 yards off the line of scrimmage]. Cato June at 230lbs and Ernie Sims of Detroit at 220lbs are typical 4-3 Tamps 2 OLBs. In the Patriots' system, these guys are too small to play the outside. If they were on the Patriots, these players would be very big strong safeties. In the Tampa-2, the OLBs count on the four defensive linemen to tie up the five or six offensive blockers so that they can use their speed to flow to plays.
Most importantly is the middle linebacker position in the Tampa-2. The big difference between the Tampa-2 and original Cover-2 is that the middle linebacker in the Tampa-2 has significantly more pass coverage responsibilities. His job in pass defense is to sink into a moderately deep intermediate zone (10-15 yards off the line of scrimmage) and pretty much play a shallow center field safety. This means the guy has to first and foremost run like a gazelle. Brian Urlacher is probably the best example of a current Tampa-2 MLB. He is responsible for way more ground than a 3-4 ILB.
So a 3-4 team playing the Tampa-2 will be run on incessently if their linebackers are prototypical Tampa-2 backers, as the edges will not be sealed because the linebackers are getting bulldozed by pulling guards and fullbacks. If the team has linebackers that are big 3-4 backers, they will be destroyed on the intermediate routes between the linebackers and the safeties as the linebackers will be too big and slow to keep up with the seam and fly routes.
There are exceptions to the player profiles with atheletic freaks who are 6-4, 250lbs who can run a 4.5 40 and have great agility. These players exist, but they are not common enough to build a defense around. If a team is lucky enough to have one of these players, then they have an edge, but the schemes are not formatted to fit the freak instead of the typical NFL size/strength/speed tradeoff.
