For most tourists, I think that 15 days is plenty. I’m guessing that Indian visitors can also get a visa prior to arrival in the country that would allow a longer visit.
Indians were specifically targeted in this rule change. I am guessing that the numbers will drop off, not so much for the limited number of days, but the additional cost of staying. Also, the hassle will be another discouragement for Indian visitors. We’ll see what happens with the numbers and I wouldn’t be surprised if there’s further adjustments by the Thai Govt.