The thing that worries me most about doctors like the OP's, is not so much the outdated, misleading and wrong advice they are giving about T2 diabetes (it doesn't take much googling to work T2 out for yourself and then ignore them), but the feeling that any other advice or instructions a doctor might give me on any aspect of health may be equally wrong. Nowadays any medic has to earn my respect.
Sally
You doctor seems to have confused beta cell dysfunction and beta cell loss. You don't lose them, some beta cells just don't work properly once diagnosed with prediabetes and type 2.
Unfortunately scientific studies are mostly concerned with drug interventions but the science of beta cell dysfunction can be followed through studies like this and the older studies referenced. http://bjdvd.co.uk/index.php/bjd/article/view/247/451
I think function can be restored and certainly damage halted and we are seeing this on the forum all time with people using various means to give the cells a restorative rest or reset. How else are we seeing reductions on HBA1C and OGTT test results? If your doctor was right it would just be progressive decline for everyone.
Unfortunately we don't have 10 plus years of post diagnosed data for people following consitently low carb, keto, post newcastle etc and science has not figured out ways of testing the quality and quantity of beta cells from the living.
Please keep going and ignore your doctors doom and gloom. Do your best to find a sustainable long term way of managing type 2 that will support/restore your beta cell function.