Although a glance at a schematic might lead you to think it is simpler than a classical (Golden Age) PP KT88, the parts are more expensive. Way more expensive, just as 300B’s (as a group) are more expensive than KT88’s (as a group).
Another factor is sensitivity to parts quality. An amp with 20 dB of feedback (which is nearly all Golden Age amps) tends to wash out differences in the sonics of different parts. This is exactly what feedback is meant to do ... 20 dB of feedback is a 10:1 reduction of all sources of coloration. A zero-feedback amp, by contrast, reveals the sonics of every single part, particularly at critical nodes in the circuit. This raises costs compared to the PP KT88 equivalent.
Depending how you feel about the sonics of solid-state and feedback, you can travel a continuum between modern Class D, with sophisticated and complex feedback, to Class AB transistor or MOSFET with 20 to 40 dB of feedback, to Class AB push-pull pentode with 20 dB, to Class A with zero feedback. Each type sounds different and has different distortion spectra.
Comparisons between modern Class D and all-triode Class A are not absurd, despite radically different technologies. Class D and Class A both skip over the many difficulties with Class AB device switching, whether bipolar transistor, MOSFET, or pentode (each device type has different artifacts associated with the AB transition). The difference is Class D switches at 100 kHz or higher, with pulse width translating to signal level, while Class A is non-switching and like a preamp, fully analog from start to finish.