Then there are new requirements like AF and optical image stabilisation. With classical prime lenses, focussing often would move the whole lens forward and backward. Or at least some large lens group. That makes the optical construction easier but prevents fast AF. Modern lens designs would use a rather small lens in the middle for focussing. With the reduced mass, the focus speed goes up. Image stabilization also asks for a small lens or lens group to be moved.
But one big difference shouldn't be overlooked either: the abundance of compute power. Today, you can easily simulate a lens system of any complexity. Leica did actually use raytracing to optimize their lenses in the 50ies - but in the absence of computers, they had a room full of people who did the calculations manually. So with a months worth of work, they managed to calculate the path of like 5 rays through their given lens design. Which had like 6 elements. Modern prime lenses can have up to 20 lens elements, zoom lenses up to 30.
For a good comparison about different lens design approaches, one should look at Leica lenses. Because Leica both still makes classical manual focus lenses for the M system, which typically have like 5-7 lens elements, and absolutely modern style lenses for their SL system, which have twice or more lens elements. In each design philosophy, they are making some of the best lenses, cost usually not the top priority :). Which makes a good base for comparisons. For example, they quite recently both introduced a new 35f2 lens for the M and the SL mount.
Personally, I am quite fond of the prime lenses with few lens elements. While modern glass coatings can pretty much eliminate reflections on glass surfaces, every single glass-air boundary does create reflections. So while the modern designs are technically close to perfect, I think those designs with few lens elements have a special image rendering. Very contrasty, lots of character.