Right, but they weren't doing that. There's no evidence they were and no motive for them to do so. The comparison with athletes is not apt. A pro footballer who bets on himself and manipulates the outcome is still a pro-footballer afterwards. A politician who bets on themselves and deliberately loses is not a politician afterwards. It does not make sense to do it.
Well except for the fact that the salary option is:
granted gradually over a year period
requires you to do a full-time job
If they would be able to get even a slightly worse salaried job instead of being an MP, then the financial motive is - in contrast to your claim - actually in favour of him losing