It’s true proper training is more important than hours.
For my considerable experience as both a student and a trainer in airplanes, I have noticed that a new pilot's receptiveness to learning is like a sponge, but backward. If you pour water (the training) over a brand new sponge (the student's brain), a lot of it rolls off, rather than being absorbed. As the student becomes more familiar with simply being airborne, the sounds, the tasks, and unusual attitudes and events, they can absorb at
retain more training and experience. I have found that the pilot with a few hundred hours of experience in a 150, or otherwise simple, poorly equipped airplane, has opened up their learning sponge considerably, and is ready to learn more, and understand how, and when to apply it. I trained a 7000 hour airline pilot in his new taildragger. As he slid the plane all over the runway as I took over to avert a groundloop, he was obviously overwhelmed. New type, with no idea what to expect, so he did not absorb what was happening. It was only when I turned around to backtrack, and showed him the S shaped skid marks he had left, that he began to get it - his sponge opened up to receive more learning. It would have been too late in real time, but when he was able to catch up, he could absorb more - then, he
began to be ready to learn tailwheel. That was his fifth hour flying tailwheel with me, he had to have the bad experience to understand what he was learning. Happily, aside from a bit of spent rubber, it was otherwise safe.
A very new pilot trained in a DA-20 will have mastered more of the plane and its capabilities in its intended operating environment in 100 hours, than that same very new pilot put directly into a DA-42. After the first 100 hours in either type, the pilot of the DA-20 will be safer in the DA-20, than the DA-42 pilot would be in the DA-42, simply because the DA-20 is simple, as its operating environment, and flying it will afford the new pilot the opportunity for settling into the environment before complicating it. With that 100 hours in a simple type, they'll be much more ready for the DA-42, because they can devote much more of their learning attention to
differences, not still building basics - their learning sponge is more receptive to holding more knowledge, rather than that knowledge overflowing the sponge.
Learning piloting is very much a stepping stone process. Yes, you can take a direct entry MPL candidate and have them right seat in a jet in a 100 hours, but it is certain that their personal learning sponge has not yet opened up. There are things that they still need to learn. Maybe they learn them quickly enough, on rare occasions, we read that a skill was not as well developed as we hope.
Yeah, the 1500 hour 172 pilot is over qualified in a light single, and if they have the slightest aptitude toward piloting, their personal learning sponge is really ready to absorb lots more. They will need more type and operating environment training, but the basics of piloting will be solidly ingrained in them, and will be "unconscious competence" for their piloting skills for the rest of their career. The 300 hour MPL right seat jet pilot may have achieved "conscious competence" but they have yet to achieve "unconscious competence", and worse, right seat in a jet flying scheduled routes is a slow way to learn, compared to other piloting environments - I hear they use autopilots and autothrottles a lot!
A pilot with 1000+ hours in whatever simple plane is easier to validate for higher training, and will likely accomplish it sooner, because that experience has given them a sound, maybe even excess, basis for learning, their personal learning sponge is open to learn more.