As moonslav indicated, we basically have him for the next 4 years for about 50 million total, so a 10 year 300 million deal would in effect be an extension of 6 years at about 42 million a year. Plus we assume all the risk of him getting injured (and he just had knee surgery). If terms like that are the best we can do, it's just not feasible.
You have to wonder, too, at what point the MLB rate of inflation starts to slow down a bit.