You don't pay for pas performance, you pay for future performance. Gronk is obviously going to be a major, major player in the next 10 years. And he got money reflecting that (I even dare say the Patriots made out in this deal).
I do think Welker should get a 2-3 year extension, but you have no choice but to think short term with him. Your not going to give Welker a 30 million deal...not at his age and point in his career. The Patriots seem content to pay him 9 million this year, and maybe even next year, but after that, Welker is a huge question mark. The Patriots are in the right for moving cautiously with Welker, and are definately in the right for giving that money to Gronk.
Whether people want to admit it or not...Gronkowski and Hernandez are the Patriots long term future. Welker is not. Locking up Gronk will only ensure we stay relevant for many years to come.