Were I a buyer, the older airframe would concern me. From what I’ve read, the early Fulcrum’s airframe was designed with the Soviet philosophy of “disposable” hardware. As such, the airframe life on those early birds were pretty low - less than 3,000 flight hours, IIRC.
That’s just my amateur opinion, of course.
Mig company is a Soviet leftover. Their lust undistutably successful design was Mig-21 and they got stuck in a 1960s mentality.
Aircraft were rather inexpensive at the time and durability of aircframes weren’t of big concern as well, because technology advanced rapidly. They build thousands of fighters in no time and in a few years they were scraped as absolete, about time or earlier than they started to fall apart.
Needless to say that it has changed in 1970s as aircraft became more complex and expensive and their approach hasn’t worked ever since.
Mig-23 was an attempt to build a rather sophisticated airplane under that same mass production concept and it wasn’t that much successful and it only went worse with 4th generation Mig-29, designed and marketed as a cheaper and smaller alternative to Su-27 by Sukhoi.