This is nowhere near the longest ban for breaching betting rules, Bradley Wood was banned for 6 years and Joey Barton for 13 months.
More recently, they banned Andros Townsend and Daniel Sturridge for four months each for the same reason, but as players are still ignoring the rules, they obviously thought a harsher penalty was needed.
It seems fair enough to me, there's plenty of other long bans for other offences, like biting, failing (or missing) drug tests etc.
In the Sixties players were jailed and banned for life for gambling on results (and match fixing).
https://en.m.wikipedia.org/wiki/1964_British_football_match-fixing_scandal