This is important. Most people in your situation have already been taxed. They will need to file the paperwork to show that, but they will owe little or no tax here in Thailand.
In my case, I get 190,000 THB for being old. I get up to 100,000 THB as "expenses" computed against 50% of the income. And finally the 60,000 THB allowance everyone gets. In total, a deduction of 350,000 THB - in other words, I would need to report more than that to even begin to owe tax. Then I get to deduct the already paid taxes mentioned above.
As @FritsSikkink mentioned, you are exposing yourself to a potential fine. How would they enforce it? If they get sophisticated enough the revenue department will inform immigration of people not paying their taxes. As @KhunLA pointed out, the banks will be reporting the cash flows. So, when you go to renew the extension....