Um, no and no. A tone setting of "10" puts the pot at maximum resistance, thereby doing its best to prevent the high frequencies taking the path through the pot and cap to ground and keeping them in the signal chain. A guitarist saying they're running "wide open" will typically mean volume and tone all set to maximum.
Given this fact, the pot's value does in fact make a huge difference. The average impedance of the input to the amplifier head is 1MΩ, so you are in fact losing quite a bit of the guitar's native high end through the tone circuit as the easiest path. At 250K that loss is 80%, and at 1MΩ, the highest-value resistor I've seen used in guitar wiring, the high-end loss only shrinks to 50%. So if you think a guitar is bright with 500K pots, imagine what it'd sound like if you used a pot that actually kept a majority of the high frequency signal.
You are right that you can always set a higher-value pot to a lower resistance. However, with audio tapers that sweet spot can be hard to find, and your suggestion of linear tapers is disingenuous; if you use them, you lose the tone all at once on the bottom half of the pot's values instead of more gradually through the entire available range, because the human ear perceives volume logarithmically; what we hear as twice as loud requires ten times the power. The difference between 500k and 400k is less than 1dB.
