At the moment there is I believe only one UK airline training MPL candidates.
I have heard directly from someone high up in the training department of that airline that the standard of pilots commencing the final phase of the course ie the company type rating training, is generally higher than those in the past from traditional backgrounds.
It really makes very little difference if an airline captain has 70 or 170 hours of light aircraft flying - remembering that a lot of those 170 hours may have been flown in training environments where real clouds are rarely seen. The ability to make decisions pertinent to command of an airliner is gained by sitting in the right hand seat for several thousand hours and participating (assuming that good CRM principals are being followed and the captain actually involves the F/O) in the decision making processes which happen throughout the day.
There is also no reason why the MPL should be significantly more expensive than a traditional CPL/IR - any TRTO booking significant amounts of simulator time will get a level D simulator for less than £400 per hour, and bearing in mind that is split between two candidates as they fly as PF and PNF during the course it equates to less than £200 per hour for their training, which is significantly less than most IR schools are charging for a seneca and not much more than you will pay for commercial training in a PA28.
Whilst at the moment there might be a shortage of instructors - in the long run there are probably enough pilots who would be prepared to work as SFIs but can't fly for medical grounds or would be happy working in a nice warm simulator to get around that problem. Airlines running the courses could even use more F/Os as SFIs than tends to happen at the moment.
Most of the opposition also comes from those with vested interests - CPL/IR holders who are building up hours trying to get an airline job, or FTOs which are not large enough or don't have airline contacts and are therefore unable to offer the course. There would also be a potential loss of income for the major schools - as the MPL needs a sponsoring airline then the FTOs could no longer take thousands of pounds from aspiring commercial pilots even when they know that there are no jobs available at the end of the course, as they do at the moment.