Yeah, it's a really fun set of tradeoffs to play with. You're correct to a first approximation. Channel bandwidth is is easier to come by at higher frequencies (a 20MHz wide channel at 150MHz would be a giant swath of the band, but a 20MHz channel at 5GHz is very small relative to the centre frequency).

But... then you start encountering higher path loss. To make up for that, you need to increase transmitter power (whether by increasing actual power, or by using higher gain antennas, etc) or by adding redundancy to the data stream (e.g. FEC). The added redundancy chips away at your bit rate, but corrects errors.

In practice, whether you get a better net transfer speed on a narrow lower-frequency band or a wider higher-frequency band is going to depend on a lot of factors. Sometimes it pans out, sometimes not.