Oz-F,
If I understand your original question, you are perhaps wondering why an airliner capable of cruising at say 0.8M (8/10ths of the speed of sound) chooses to do so at 35,000ft rather than at 5,000ft, where its true airspeed would be higher.
The answer is that it might be possible, but it wouldn't be economical. Air density increases as altitude decreases. As an exaggerated analogy, think how much more effort you expend walking across a swimming pool waist-deep in water, compared with walking along the poolside. Likewise the engines of an aircraft would need to produce more thrust at low altitude even to fly at the same true airspeed. Add to this that jet engines are more economical (low specific fuel consumption, ie less 'litres per 100km') at higher altitudes, and this helps explain why airliners climb to cruising altitude as quickly as possible, and stay there as long as they can.
Aircraft designed for high speed/low level are of course optimised for this with such features as highly-swept wings, buried engines with clever intake design, and engines with reheat (= large fuel bills).
A moderately-swept wing such as on most airliners experiences a large increase in drag as Mach number gets close to 1, and it just won't be able to accelerate; added to that, the engines need to breathe subsonic air, and a simple intake on a large-fan podded engine is not suitable to provide this.
Hope this is of help,
Rhys.