It doesn't work exactly like that. The ECR reduction per game is 1% of the current ECR meaning that if you are at 50% ECR and you play a game, it will be reduced by 1% of 50% which is 0.5%, so your resulting ECR will be 49.5%.

That means that if you start at 100% ECR you can play 28 - 29 games before you get down to 75% which will regenerate in 1 day. If you play 100 games, that will bring your ECR down to around 36%.

The end result is that after ~29 games / day you won't earn any more DEC, so a bot that plays 100+ games / day will earn roughly the same amount of DEC as a player of equal rating that plays 29 games / day.

So as you note, there's no incentive as far as earning more DEC to play more than ~29 games / day, however if you can increase your rating by playing more games then that is well worth it because it will increase your future DEC earnings exponentially.