[sceptical]

Forcing Russia to allocate a large amount of its limited air defences to defend Moscow seems like a good result in itself. It appears to allow Ukraine to attack airfields, fuel storages and other infrastructure inside Russia with good results.

[sschueller]

Why would a state as large as Russia have limited air defense?

[Arnt]

How could it be otherwise? Seriously. Russia isn't Singapore, even a single line along the front world require enormous amounts of equipment.

[rsynnott]

Russia is physically large, but pretty poor (its economy is only about the same size as that of Italy) and its defence industries are increasingly cut off from essential supplies. Russia has limited everything right now.

As we’ve seen over the last couple of years, well-trained competent military personnel are also in short supply.

[sickofparadox]

Why? No clue - probably gross incompetence and corruption. The fact is that they have been at war for two years against what was one of the smallest militaries in Europe and still have not managed to obtain air superiority, indicating that they have some kind of shortage in the AA department.