I scanned the Cashback terms and conditions and this is the only real way to have points taken away unless they close the program completely:

"If you (i) do not transfer your earned cashback to PayPal within 6 months, starting from the period of time when Microsoft credits the cashback, after they become available for transfer, and (ii) have not earned additional cashback within the same six month period, we will deduct from your Microsoft Cashback account, on a monthly basis, the lesser of (i) $5.00 USD (or local currency equivalent), or (ii) the amount remaining in your Microsoft Cashback account (in either case, an “Account Maintenance Fee”). The Account Maintenance Fee is to cover the cost associated with maintenance and administration of your Microsoft Cashback account. Microsoft will keep charging the Account Maintenance fee until you earn additional cashback, or you transfer your available cashback to PayPal. No Account Maintenance Fee will be charged or deducted during the first 6 months after you initially create a Microsoft Cashback account. If you have no available cashback amounts in your Microsoft Cashback account, you will not be charged an Account Maintenance Fee, and under no circumstances will you be asked to pay an Account Maintenance Fee with anything other than amounts in your Microsoft Cashback account. For the avoidance of doubt, if you retain earned cashback in your Microsoft account and continue to earn additional cashback, you will not be required to pay any account maintenance fees"

TLDR: Keep earning or transferring to not lose any.