should i just not bother renewing wtc and ctc

thor

I reckon you are in a lucky position. If they keep rolling it over every year then as long as you don't spend it and have stashed it away in a savings account it will be earning an ever increasing amount of interest which you can do with as you wish. This is what credit card stoozers dream of - an interest free loan.
Also there is also the possibilty that they may never ask for it back and let you keep it as I'm sure that they have done so for some cases where the overpayment was their fault(despite being told repeatedly that they had messed up by the recepients). You can't lose so take advantage of it.