No ordinary turbojet?
Lewis writes: "Ordinary turbojets are limited to around Mach 2.5 in normal use, though they can beat Mach 3 if the user doesn't mind replacing them afterwards."
True for almost all turbojet aircraft, but don't forget Kelly Johnson's SR-71, a.k.a. Blackbird: officially Mach 3.2+, which most definitely did not require new engines (J58s) after each flight. To be fair to Lewis, Kelly's design cleverly bypassed the majority of the airflow at the higher speeds, reintroducing it into the afterburner stage (i.e. at higher speeds it was mostly working as a ramjet!). In short, Mach 3 and above can be achieved with traditional turbojets, but it is very tricky to do properly (plenty of documented history exists on the design and operational challenges of this aircraft, majority of which surrounds how the engines behaved at supersonic speeds.)
One interesting point to note is that fuel consumption (expressed as fuel used/distance covered) improved with the higher speeds. I have attended a lecture by a former Blackbird pilot who claimed that if they were running short of fuel, e.g. due to extra in-flight manoeuvering, they would accelerate the aircraft to ensure a safe return (and get back sooner to boot!)
I suppose similar consumption figures must apply for scramjets...