Question on dishwasher

[Steve Tom]

Hello fellow landlords.

My new tenants have been at the rental for over a year. They are quiet and nice. So far there has not been any complains or issue until today. 

The rental unit has an original dishwasher. It is 15 years old but was rarely used all these time. But assuming due to plastic components and its age, the tenants informed me today that for the last few uses the dishes are not as clean, and they also noticed there is now rust left on the silverwares.

It is an old dishwasher, but is otherwise functional. They even willing to look for a replacement (not sure if they will pay for it and then take away with them after the lease expires next year).

if the dishwasher is fucntional, should i replace it? I can replace it and leave it for the next tenants. or argue with them that it is funcational and does not needs to be replaced. (I am leaing toward replace and so i don't have to do it again for a few more years, but wife wants to save money.)

Thank you.