SAAB has a monopoly when it comes to fighters as well, there's a very strong tradition in Sweden that it produces its own fighter instead of importing it., SAAB is the only one with the resources. Very expensive, but that's the way Sweden neutrality works/worked.
Different specialities, yes, but they overlap. MiG/Sukhoi both fighter design, with Irkut fighter production. Sukhoi, Ilyushin, Tupolev and Yakovlev are all producing aircraft for the civilian market. Yak and MiG both trainers. Sukhoi/Yakovlev both aerobatic/basic trainers. Tupolev/Sukhoi both bombers. I see a lot of overlapping areas where the parties can share knowledge. Also don't forget that a lot of the specialised knowledge came from shared research institutes. I think joining them will create a strong international aircraft builder. Of course Russia will buy its own fighters, just like Sweden, but on the commercial market and export market a lot can be said for such a merger. Aeroflot is buying non-Russian, Yak-40s get replaced by western VIP aircraft, and the transport market is lost to western companies and Antonov. MiG can't compete, Sukhoi can, but the Sukhois are too big and expensive for most markets, so they lose market. I can see why the Russian government wants a single strong aircraft builder.
And under the Soviet system, the selection of a fighter was always based on design specs and bias for a particular OKB, not price, so instead of three OKBs putting forward three designs and one voted favourite on political grounds, now the giant can put forward three designs and they can be judged on their specs, and the air force can even say, combine the three designs into one good one.
Even though it can use its power as monopolist, by forcing a design down the air force's throat, as a powerful monopolist company can do in the west. I don't think that can be done in Russia, politics is too strong. I assume that the state will be the biggest shareholder. They will just fire the board and get new directors and management in, which does things according to the government's liking. It's always been this way with the different OKBs anyway (see Tupolev's career).