One hit will create thousands of fragments. How many consecutive hits would be necessary to create severe enough Kessler event to destroy most of the Stralink satellites? Dozens?
Space is big. The average breakup seems to create about 300 fragments [1], so if Russia blows up 100 Starlink satellites that's 30,000 bullets that have to randomly hit another 3500 objects spread over an area roughly the area of the surface of the earth. Except that it's worse because space is 3-dimensional and most bullets will spend little time at that altitude, and eventually deorbit. Over a decade it will do a lot of damage, and at some point things will escalate to a point where it's hard to handle. But Russia needs results within weeks or at most months.
1: https://en.wikipedia.org/wiki/List_of_space_debris_producing...
Causing a chain reaction is significantly more difficult then people think.
Satellites are usually in orbit, not randomly in a space, so your target is to make a mess from that given orbit, not to try to mess up whole space.
The only preventative measures which would've worked would be diplomatic and economic counter-pressure from the UN and more specifically Europe and other NATO allies, or substantial military build up capable of seriously endangering US conventional forces.
Ukraine would be in a much different military position today if it were a pariah state, as opposed to a nation declaring a strong intent to join both the EU and NATO. Turns out diplomacy, allies and soft-power are actually kind of useful things when you're not the dominant military hegemon.
Spoiler: it has never worked. What you are thinking of are diplomatic, economic and military moves and counter-moves or negotiated settlements but the aggressor is always forced to compromise or delayed such that they lose their advantage.
Appeasement - the aggressor making demands and getting them with the idea that they will be then be satisfied - has never worked.