@antonioGr,
Apologies for the long post back, but I hope this can clear up some of the confusion / concern. As a little background, I too shared significant concern over the MPL during my journey to join an FTO. I researched it quite a bit prior to jumping in the deep end and applying.
I was told by someone on the ATPL integrated course that MPL license holders can only fly as first officers and cannot be upgraded to Captains in the future.
Where is his source for this? Having previously applied for the easyJet MPL and Virgin MPL prior to joining the ATPL at CTC I have never once heard this mentioned. There is a lot, and I mean a lot of stigma towards MPL licences among people who prefer the ATPL. However, I compare this as being no different to a passenger swearing by one carrier and never trying another despite all the convincing in the world - creatures of habits us human beings.
I would take the "someone on a course told me" or "someone I know on a course has told me" with a pinch of salt. Being at an FTO now I can tell you now the places are rumour mills!!
after you reach 1500 hrs you can convert your license into an ATPL (unfrozen) there is a so called "multi-crew" limitation on your ATPL license which does not allow you to become Pilot in Command aka Captain.
Last time I checked being a captain still requires you to work in a multi-crew environment. The difference between the MPL and ATPL is that you do not conduct any single pilot training, or as much therefore and thus are not considered qualified for single pilot commercial operations. This explains the restriction. But, if you plan to fly for easyJet or another MPL operator you'd be in the multi-crew environment for which the very licence was created. I can't see it being an issue.
MPL training is generally shorter and less onerous. However, MPL graduates are effectively restricted to the right-hand seat in the cockpit to fly only as First Officers. To become a Captain, the MPL holder would still need to acquire an ATPL.
This isn't all that different to an ATPL. In a nutshell the MPL delivers airline training at lower costs to all parties involved. An MPL cadet saves tens of thousands of pounds on programs such as easyJet's MPL versus the equivalent whitetail cadet joining the same airline who would have to pay their own type rating. Training that isn't appropriate to airline operations is taken out of schedule altogether. Namely, some single pilot ops mentioned above etc.
But back to your point, an MPL graduate is restricted to the RHS as much as your frozen ATPL graduate would be. Each require 1500 hours at a minimum before they can 'convert' their MPL to an ATPL or 'unfreeze' their frozen ATPL for it too to become an ATPL. At this point, both would then have an ATPL licence and I can presume, any pilot with the aforementioned and the hour requirements for any position can then go on to apply for command at the airline. The difference, as I touched on above, is the individual converting their MPL would, by the very nature of MPLs training, be 'multi-crew operation' restricted meaning that flying the line in your easyJet A320 wouldn't be an issue, but being a one man pilot in your single-pilot private jet as part of a commercial operation would be be.
As a side, easyJet is very heavily invested in the MPL and was one of the first carriers to get involved. In my mind, they have faith in it and now have employees flying the line with one too. I wouldn't be too concerned, as at the end of the day the Generation easyJet program is a great one.
Hope this helps.