A more apt comparison would be Trish cr.L snd Iron Man cr.L if we want to look at chainable cr.L. Trish is essentially identical to what we have now of Cap only it's 5 frames and chainable. Same for Iron Man, he has more range but less block advantage. Ryu's cr.L is another good one, + on block, chainable but 5 frame.
For sure though if you make Cap's cr.L +2, 4 frame and chainable it becomes the best cr.L in the game. If people don't want buffs to his cr.L then give more push back on Shield Slash M/L on the ground so he has a legitimate block string.