Second, they have made really serious modifications to it, several of them. In both Ukraine and Russia. The latest one includes new wings and an electric engine.
Over the years there were several planes that were supposed to finally replace the old An-2 but none of them actually displaced the aircraft because they weren't as easy to repair or they were too expensive or they didn't offer the same short field operational capacity.
Various suggestions from just replacing the old engine with a new model An-3 through to all sorts of actual new designs all failed for one reason or another... the most recent model was an An-2 made of composites and with much better performance but was too expensive to realistically do the job and likely would not be as simple and easy to keep running.
Until they get batteries that can cope with the cold electric is not going to work either.
The new LMS-901 and the other bigger plane look viable, but An-2s are going to remain in use in backwaters till they crash or can't get parts any more.
The Military however needs replacement aircraft and the two put forward.. LMS-901 and heavier twin engined type, are probably mostly held back by lack of suitable domestic engine in that power range.
The VK-800 should fill that gap.
and producing An-124 may not be a long lived initiative, specially because it could not be used to serve the international market.
The smaller lighter An-22 was a very useful aircraft with a useful payload capacity and good flight range... I would think an 80-100 ton payload aircraft would also be very useful too while being cheaper to operate than the much bigger Antonov.
I would think getting the factories wanting to make An-124s adapted to making Il-106s would make rather more sense.
If they start with the Slon, PAK-VTA and new Il-76 the need to bridge a gap with the An-124 may not justify a lot of new airframes.
If the PAK VTA is the Il-106 then I would think production of the Slon and the Il-106 would actually make the An-124 a bit redundant with no new airframes being required.... the Il-106 would be cheaper for lighter loads and shorter distances, while the Slon would allow the same payloads over greater distances or heavier loads over similar distances.
The engines used are not particularly fuel efficient nor amazingly reliable.
You are right about eventually the il106 and the slon will be produced. The problem is that this eventually means at least 5 to 8 years.
What is holding them back is their engine... the PD-35, but that same engine would be used on an upgrade for the An-124 while a new PD-24/25 optimised to replace the older engines likely would not be used on anything else and while they would improve reliability and fuel efficiency there are no plans to use them on anything else which makes them a bit wasteful...
PD-35s on the other had will be used on SLON and Il-106 (4 and 2 engines respectively) as well as on the Il-96 upgrade (2 engines), which makes a lot more sense moving forward.
Furthermore, concerning the current situation of Russia with the west, a Ilyushin aircraft will face the same restriction as a Russian produced An-124.
The Il-106 and Slon will be Russian designs with Russian parts and engines... they are the solution to problems getting western parts.
Soon Zaporozhye Alexandrovsk will not be anymore in Ukrainian control. If the factories are all destroyed then the issue of the production of the old D18T engine (a decent stopgap while they wait for the new russian generation engines) can be easily solved.
A big part of the new aircraft is no longer needing the Ukrainian engine.
Furthermore the inheritance and intellectual property of Antonov and motor sich (together with zorya and with most Soviet firms based in Ukraine) can be transferred back to Russia once they sign the capitulation of Kiev.
But why... the aircraft and the engine are dead ends and the engine needs replacement more than the aircraft does.
Eventually the west will not have choice but to recognise it (as they will also have to recognise Crimea as russian and the Donbass as either russian or part of the new novorossian state).
There is nothing Russia could possibly do to compel the west to recognise anything at all.
And if the west will not recognise Antonov aircrafts build and operated in Russia, Russia can in 5 years forbid operation of Boeing and Airbus airplanes over its airspace (and possibly even over the whole CIS airspace).
And if they respond in kind then airlines from the rest of the world are going to get pissed at needing to buy both western planes to operate western routes and Russian planes to fly over Russia... and the bans will mean flying to the EU or from the EU to Asia can't be done over Russian airspace because if you use western planes you can't fly over Russia and if you use Russian planes you can't land in the EU.
And for what... so Russia can keep using some Ukrainian planes?
The existing types will be fine for the next 5-6 years and then Russian planes can start to replace them forever.