Ahenobarbus would never do that. He was an aristocrat and would obey the proper form of things: if he becomes severely ill or dies, his most senior legate would take command and await senatorial confirmation or replacement. This senior legate could never be a young man who'd never even been quaestor; it would be a man of consular, or at least praetorian, rank.
If Gracchus was such a good soldier, he would have stayed in that area; it's more than likely that he enjoyed, and was all in all better with, the legal and tribunal spheres.
Most soldiers were well off, and Gracchus's Reforms wouldn't help them much. Why would they blindly follow the promises of an aristocrat and rebel against the Senate? Gracchus's main reforms were to redistribute land among the Roman and Latin Head Count, and to enfranchise the Italians; pre-Marian soldiers were all propertied men with lands and businesses, how could Gracchus's changes help them much?
For the soldiers, it's a high-risk low-reward scenario. They can be defeated and killed, or deprived of citizenship and crucified or thrown off the Tarpeian Rock, and all their property confiscated and families left destitute; or they win and...maybe double their holdings after much civil strife and bloodshed, and work their asses off for the benefit of some lazy bums and non-Roman Italians (in their eyes).
Africanus and Aemilianus, and Gracchus, are two very different things. The two Scipiones were more centrist, and their commands were merely used to defeat an enemy and gain glory and gold for themselves (and the Treasury); they had most senators' support.
Gracchus would spend Treasury money and Roman ager publicus (in Italy, no less!) to enfranchise the poor; well, the senators could never stand for that! For one, more men in the Classes (because they're now propertied) means slightly less than complete domination by the Senate. Secondly, less ager publicus means less leasing contracts to the equestrians by censors (bringing in a lot of money for the equestrians who farmed and ranched on the leased land, and Treasury).
Also, everything the tasty fox said.