it feels like one additional year of math education (which builds upon previous math knowledge) allows to modell (and thus understand in depth) significally more phenomena and structures than the previous years.

Since math professors don't look like bodhisattvas, I rather suspect there is a turnover point when the marginal utility starts to decrease.

Generally speaking, when you start learning an unfamiliar skill the first steps have close to zero marginal utility and only when you can actually achieve something does your utility increase. Once you achieve competence, however, I doubt that your marginal utility will continue to increase.