I'm not familiar with your home town, but if it is anything like major cities in teh states and in Asia, the witch's brew that passes as atmosphere will cause anything rubber (and more things bubber-like) to dry, crack and cease functioning pretty quickly . . . . (months, perhaps, certainly a small number of years).
Oddly, riding seems to prolong their usseful life.
Teh best advice regarding the bikes (I am in no way competent to give relationship adivce, likely niether is anyone else here) is to ride them, at least as long as it takes them to wrms up comepltely, as often as possible. Once a month seems a reasonable compromise . . . .
If you simply cannot carve out an hour or two amonth to ride, you should, in my opinion, admit to yourself that you are out of the riding business, and sell the bikes.
Best of luck!
"Here is your country. Cherish these natural wonders, cherish the natural resources, cherish the history and romance as a sacred heritage, for your children and your children's children. Do not let selfish men or greedy interests skin your country of its beauty, its riches or its romance.” T.R.