You haven't mentioned which type of visa your if has, (Presumably she is here on a settlement visa or FLR?) but whichever it is, I don't suppose it matters.
When her visa expires, the company she works for would be liable to big fines for continuing to employ her. £10,000 per employee, I think. Your wife would also be in breech of immigration laws and will be sent home, when she is caught.
It's also not what might happen immediately but what would happen further down the line. If you allow her current visa to lapse without a new visa, I doubt she would be allowed back into the UK for quite some time after she goes home to Thailand.
If she is on ILR for her next visa, or even FLR, you have already made quite a big investment towards your wife staying here in the UK.