Huh?! How is it holding back the design and fluidity of the game. That sentence doesn't even make sense. Having a Block button is simply a better game mechanic. You want to block, you press a button, it's the thing that makes most sense from a gameplay point of view and it is also the most intuitive thing to do. It doesn't make sense to use directional inputs on a controller to perform an action. Directional inputs are for moving, buttons are for performing action. This is not limited to fighting games only. For example, In Metal Gear Solid 2 at the very end where you have to fight Solidus Snake and all you have is the High-Frequency Blade, in order to perform any hit with that sword you had to use the left analog stick. That was an incredibly frustrating fight because of that one bad game mechanic. It doesn't make any sense to perform attacks with directional inputs, it's just awkward and counter-intuitive.
I have played fighting games with both a Block button (MK, Soul Calibur, Virtua Fighter) and without (Tekken, Street Fighter, Injustice) so I have plenty of experience with both. It's not a matter of sticking to tradition, it's simply a matter of which is the better mechanic from a gameplay point of view, and that is the Block button.