Most Thais I know already gamble online - there are multiple Line Groups and betting is on things as diverse as Football scores and the closing numbers of various Bourses across the globe. Its called Hanoi Lottery. Most lose but I have had one friend win big (and bet big) only to be denied a payout when the Group closed.
Don't kid yourself that this is not happening, unregulated, right now and that many Thais are becoming poorer for it.
If it is legalised there may be some protection (for the winners who when they win big often find the site closed and no payout) and the Government might collect some revenue - but on the other hand legalising online gambling may make things more accessible and the problem worse - but hey which Thai doesn't already have Line?
This is a bit like pretending Prostitution doesn't occur in the Land of Smiles.