Active low-pass filters --- good to what frequencies?
I believe your analysis to be good. I've made sallen-key 4th order filters that cut-off around 3 MHz with absolutely no worry about performance. I don't see that 10 MHz is unachievable.
It's all about op-amp choice. For a unity gain stage it's easy to ascertain where the gain starts dropping below (say) 0.99 and regard that as the limiting frequency. On the other hand, the output impedance of an op-amp usually gets worse as it enters the MHz regions so you have to be sure it can deliver the peak current without clipping or going too sloppy.
You also have to consider slew rate limitations but, as far as I'm aware, that's about it.
It's quite possible that The Art of Electronics, 3rd Edition didn't make any updates on that section since it first came out in 1980.
My first question: really? A mere 100kHz is already too high for active filters to be practical?
No, 100kHz is nothing, but it all depends on the opamp. At some point the Gain Bandwidth Product is going to cause problems. If you had an op amp with a 1MHz or 10MHz GBWP (which may have been typical at the time of the first edition of AofE, maybe they didn't update it is my thinking so I'd compare editions) then 100kHz doesn't sound too unreasonable, because you'd only get a magnitude or two of filtering and then the bandwidth goes below unity gain. Then your lowpass filter looks more like a bandpass.
Are there any other practical issues that I'm overlooking? Am I being realistic if I want such an active filter with cutoff in the order of a few MHz? (pricing is not an issue --- if I need an op-amp in the $10 or $20 range, that's fine)
If you really do need filtering past 50MHz then parasitics need to be modeled as ESR and ESL in capacitors will start to affect the filter poles and create their own filter poles at high frequencies. Use a spice package if possible. Make sure the GBWP is high enough, these days it's not hard to get op amps that work in the +100MHz range.
The main problem with that Sallen Key topology at high frequency is that the output impedance of op-amps rises, so fails to control feedforward of the input signal through the 2C capacitor, trashing the stopband.