There are diminishing returns. My extrapolation did not take into account the possibility that TCU would hold auditions for marching band or otherwise try to limit its size. If the band started to grow towards 400, I would suspect that TCU would try to limits its size to 250-300 via auditions. They might, perhaps, even relax the music major scholarship requirements.
Based on discussions I overheard between the then-music dean and other higher-ups (maybe Koehler?) during the Sun Bowl week, I learned they anticipated that the band would balloon to over 250 after the Sun Bowl if we continued winning.* This was based on their experiences with the Bluebonnet Bowl and the Independence Bowl. This did not happen. For one thing, then associate director of bands Velez had run off a number of students during the extremely hot summer of 1998. I recall some music majors even gave up their scholarships. The next year with Youngblood also caused some casualties, including me. After having ruminated on this subject for many years, I would guess that a lack of road trips to relatively nearby schools probably played a much bigger role in limiting the size of the band than anything else. It was a cheap way to travel, get out of town for a day, and see other schools.
* I just happened to end up sitting next to these guys on the plane trip out there and on various bus trips around El Paso.