IMHO, a super performing MCU, as we have with T4.1, would deserve the best performing audio Codec on the market (even if people over 16y old cannot hear anymore the difference).
Two issues with those chips are their cost and power consumption, not just of the codec chip but also the support circuitry needed to achieve those specs.
The really tough part of that support circuitry is power, and especially grounding. Even with noise in the -85dB range with SGTL5000, we regularly hear reports on this forum where people are experiencing ground loop issues which they can actually hear, which I'm guessing means noise in the -70dB to -50dB range (in the final result reaching their ears as a practical matter of how the board is actually, even if that noise technically didn't come from the codec chip). Even if when those sorts of problems can't be heard, I'm pretty sure they exist but are likely in the -90dB to -70dB range.
I have many times considered making a high end audio shield. In fact, there was one designed years ago with a high end Cirrus chip and its own power supply with the I2S and I2C signals isolated (I believe those isolation chips use capacitive coupling internally). The main problem is that all adds up to exorbitant cost. As I recall, that PCB was pretty large too.
The commercial reality of today's microcontroller and SBC dev board market is a very competitive landscape populated with many low cost products like Raspberry Pi and ESP32. Teensy occupies just a tiny niche in that market, already more expensive than many of the popular competing boards. We have to keep prices reasonable to stay in this market.
While it's fun to dream of a product which could offer -120dB level noise+THD performance and come with the power supply & isolation needed to actually achieve it in practice, the reality is such a large and expensive board would have very low sales. PJRC just can't put the financial investment into that sort of very expensive high-end audio product.
But like the CS42448 boards, I might do more on a DIY design. Maybe...