Well I think a really big factor is going to be Checkmate and any MiG equivalent because they are being designed from the outset to be modern stealthy fighters with internal weapons bays that are 20 to 30 million to buy and probably 4-6K dollars per hour to operate... now if they design them properly so they can for instance effectively deliver dumb bombs with the precision of the gefest & T type navigation bombing system they would make excellent COIN type aircraft too and affordable to many up and coming countries.
It is pretty clear with the separation of Russia with the west that resource based wars are going to start happening more often because the west won't be able to keep its comfortable standard of living and also buy everything it needs.... so what to do with that enormous military the US has... use it or lose it I would expect... it will either cut costs but get used to earn its keep or it is going to be fund starved out of existance... I suspect the former... which means most countries are going to want lots of MANPADS and lots of ATGMs and other defence items that are small and mobile and portable and most importantly effective.
Many of the older aircraft have room for upgrades to keep them affordable but to also make them more effective... I mean four wing pylons added to a MiG-15 that each carry four Igla-S or Verba type MANPADS for use against enemy attack helicopters and light CAS aircraft or incoming cruise missiles or standoff weapons...
You have to look at the location as well too... Russia is not likely going to want to use MiG-15s for anything except target practise, but some little country in central or south America where the enemy has an F-5 or perhaps Super Tucano then an upgraded MiG-15 isn't a terrible choice and relatively simple and cheap to operate, which was one of the reasons it kept being used so long.
Aircraft are delivery platforms for weapons so as long as the weapons are good and effective and not too expensive then even an older aircraft can be useful in some roles.
As mentioned the Su-17/22 has a decent payload and can carry modern weapon types. It isn't amazingly fast and it isn't amazingly manouverable... for CAS the Su-25 is rather better even if only because it has two engines so it can often survive a MANPADS hit and get home...
The Soviets found the Su-25 to be significantly superior than the Fitter in the 1980s... the Frogfoot could get airborne from an airstrip a quarter of the length the Fitter needed, and while it was slower it was better able to find targets and hit them and its performance manouvering in the mountains was rather better too.
It is not a super manouverable plane of course in a fighter duel an A-10 would win most of the time but then using Vikhr missiles the Su-25 would probably never engage in a turning dogfight with an A-10 in the first place.
I honestly think the Su-25 is a much better design... the A-10 is just too big and its weapons capacity wont mean much in combat because very quickly a MANPAD hit will mean it is heading home anyway.
Interestingly the Ilyusion competition for the Su-25 was actually closer to the A-10 in size and capacity, but actually had a tail gunner with a twin 23mm cannon and wing mounted bomb bays.... but was rejected for the smaller lighter faster Su-25.
Apart from being free the previous generation of aircraft don't really offer anything the newer aircraft don't give with improvements.
The MiG-23 is a good interceptor and a reasonable fighter because of acceleration and systems, and the MiG-27 is a good strike aircraft with a very good gun and good air to ground payload capacity, but later members of the MiG-29 family combine reasonable interceptor capacity with good fighter capacity and light strike capacity too... all in one more modern cheaper to operate and maintain aircraft.
The new air to air weapons for the 29 are vastly superior to the MiG-23s and the air to ground ordinance is improved too, so as they wear out it simply makes sense to scrap the older planes and buy newer ones, with the new ones being fully multirole.
The only real exception would be MiG-25 and Mi-24 in my opinion but drones are taking over for recon in poor countries as well as rich ones because they are cheaper... and the Mi-24 offers something a bit unique as well, but might be more interesting to paramilitary forces like police units that want troops and weapons carried on missions, or for extraction or delivery missions of small groups in or out to remote locations... like up the side of a hill or the middle of a jungle or desert.
Although mig-15 was still flying, never scene any pics of the su-7 which was a bad aircraft anyway.
The Su-7 was an excellent aircraft for what it was.... it was small and light and cheap and handled very well... it just lacked range and payload... problems that were solved with swing wing modifications in the Su-17.
It is a sort of Jaguar type aircraft that is not really a fighter and more of a light strike aircraft.