What happens to my marriage visa if my Thai wife passes away?

The thing is you're NOT 80 years old you're in your late 30's so talking about what happens if your wife dies when your 80 is nothing but pure speculation :/

okay, lets get back on TOPIC.. The person is in their late 30's SO any talk of O-A visas or retirement visas/extension is about 20 years TOO soon,.

As far as their question IF they are on a valid year-long, multi-entry VISA from a thai consulate based on marriage to a thai and the spouse dies they can continue coming and going every 90 days on that visa until it runs out

AND

If they are on a yearly extension of stay based on marriage from an immigration office and the spouse dies they can stay until that extension expires and then they find a new way to stay.

Depends if you choose for the Non Immigrant O-A visa or just

Extension of Stay based on being over 50 years of age (easiest option)

As you apply for a 1 year Extension of Stay at the time, the maximum you can stay is 1 year, until the last day of that Extension of Stay.

Your age has nothing to do with that, you need to find another way to stay inside Thailand.

Stay is as long as your current permitted to stay date.