Not that we think there's a coordinated plot here....
….but we’re entirely sure that at least some will have noted the implication, effect even, of such a ban:
If we ban second jobs for MPs now, we’ll soon wonder how they were ever allowed
Just for the avoidance of doubt, no, we’re not stupid enough to think that morals in Parliament - even if we just restrict ourselves to fiscal ones - are quite and wholly everything we should collectively be desiring.
Switch, for a moment, to the demand common enough in America these days. That no serving politician should retain control of their own money while in office - blind trusts all the way. Even, that Donald Trump should have entirely sold out of the family business. The effect of this is that anyone who has either the drive or fiscal acuity to build a business is barred from office. Anyone who has been successful in a capitalist - largely - and free market - OK, again only largely - economy is to be dissuaded from taking part in the ruling of a largely capitalist and free market economy.
Back to us here with Members of Parliament. A ban on any outside work would mean that anyone who had built a career outside politics and wanted to continue with it after politics would not be allowed to do so. Cincinnatus would not be allowed to go back to his farm because he’d not be allowed to maintain it while in office.
We end up in the same situation as that theoretic American one. Anyone who has been successful in that real world outside the halls of power is at best dissuaded and at worst banned from being part of the ruling caste of that outside world.
This being something which we rather doubt will increase the quality of the running and management of that outside world. We’d be left with only those who decided upon politics as a full time career and also only those who had no knowledge of anything other than politics. This would not be an advance.