Raising the 21 million limit is against the interest of anyone who is holding coins as it would devalue the currency. So it's fairly unlikely that the majority would agree to do it I would guess. All miners MIGHT (although it seems unlikely) as they potentially stand to benefit, but they would still need to convince all the other regular non-miner users who would be hurt by the change. Seems pretty unlikely, unless there's mass centralisation of the system (ie. noone running full nodes, everyone using 3rd party wallet services, who could collude to make it happen).