Hasn't Bali always been cheaper than Thailand?
Oh, and Bali certainly cannot be called "peers" of Thailand.
Thailand is an upper-middle income country, like China, South Africa, Mexico - poverty exists, but you wouldn't call them poor countries. And it's a whole country.
Bali is just a touristy island in a lower-middle income country (that's countries like Cambodia, Laos, Myanmar - countries that you would call poor).
https://worldpopulationreview.com/country-rankings/middle-income-countries