I'm a vet - diabetes is relatively common in dogs and cats. Probably becoming more sore because of better owner awareness/diagnosis. Most patients/owners cope suprisingly well. Cats blood glucose can rise significantly with stress -eg. chasing it round the house with a lancet so not really worth trying to do a blood test at home to diagnose - better to get a urine sample - can get special cat litter to facilitate this or take it to the vet who can do a blood glucose and also a fructosamine - the equivalent of hba1c, therefore eliminating the stress factor.
Kidney disease is a very common cause of weight loss and increased drinking in older cats so most vets would test for this at the same time.