...well, you know: The kind of contracts for such support stuff is not the "lifetime" type but something that might last for 10 years. Maybe more, maybe less. And if one of the two parties involved in the contract uses a legal "announcement" which is in a legal timely line to the end of the contract... no way to sue them.
They might have had support deals for a ten-year period. And then, IBM just simply doesn't extend them. So that's it. And about sueing IBM: If it's Microsoft who has the best marketing staff on this planet, guess who has the best lawyers and contracts in universe...