The only reason i see is that the ground is getting smaller and it is really hard to control the flow of runs with the rules to restrict fielders outside the circle to favor the batsman and as you said with all the video footage and the technology we have it is easier to learn their skill set and variations and if the bowler did not change according to that then he will find it hard to cope up with the modern game.
I do not think spinners can be effective in T20 matches as they need an old ball to spin and a dusty picth. It can be only achieved in a test cricket or later in the later part of once day cricket.
The above mentioned tech might be beneficial for the batsman for few overs but things are always different when you are actually playing on the field.
Have to agree with vishnu and eaLiTy here. Technology, Short boundaries, Power play are important factors and all of them favor the batsmen in modern cricket. Majority of finger spinners and off break spinners are pretty much useless in limited over cricket but wrist spinners such as Mujib, Rashid khan,Kuldeep, Chahal, Aadil Rashid, Zampa are still very handy and they can dominate the game if boundaries gets slightly bigger let's say 70- 75m
