The resentment comes from the sheer volume of numbers.
Tourism spend is actually down, despite last years massive number of arrivals (almost record breaking). One or two credit card companies have said spend was up, but that doesn't give an overall figure. Spending with inflation will always look rosier.
Thailand also used to gain 2000 Baht from issuing visas to Indians. They removed that. Try working out 2 million times 2000 baht. That's a big hole to fill.
The issue if you allow in so many cheap tourists, whilst you may think it looks good, you then see the knock on effects - with many looking to work, over stay, but crucially you cannot get past the low spend and behaviour. You are changing the landscape.
Malaysia recently reversed it's own visa policy for Indians, taking a more strict approach after many over stayed and began to work.