I think the OPs question may be more related to the effect of lift dumping on deceleration, rather than the complicated area of friction vs. tyre pressure.
If you didn't have spoilers, on touchdown there would still be a significant amount of lift generated by the wing. This means the maximum braking force would be limited by the weight on the wheels being a lot less than the mass of the aircraft, leading to a slower deceleration.
A lot of overruns have late/no spoiler deployment as a prime cause, especially on wet/contaminated runways.