Apart from anything else, in a 2-crew aeroplane, if one of the pilots becomes incapacitated: which has happened quite a few times over the years, it becomes a single pilot aeroplane for the duration. I'd rather trust somebody with 300 passengers in a (suddenly) single pilot aeroplane who genuinely has a fair number of hundreds of hours commanding single pilot aeroplanes than the chap who has no real experience of that nature.
One of the arguments for MPL is that in recent years most serious accidents in big aeroplanes were down to poor CRM. Technically this is true, but quite probably because the pilots up to now have had the handling and problem solving skills which meant that CRM was the only thing left for them to screw up on a regular basis. The MPL will re-introduce all the potentials for error that had been eliminated previously by hour-building.
So far, the only airline professional who I've met who really seemed to believe honestly in MPL was a recruiter for BA who had a background in accountancy then HR. I've not met a single professional pilot who thought well of it. I also hang around with quite a few aviation safety researchers, who are all fairly bemused at the idea whilst so far as I know nobody has published any peer-reviewed research into its supposed benefits or demerits, so technically we're all just guessing (but especially those who claim it will work).
G