I think 'tank' is defined by role, more than physical attributes. For example, doctrinally the Strv-103 (S-Tank) is a tank, not an assault gun or tank destroyer, even though it has no turret.
Also, when we move into the realm of hovertanks or counter-grav or whatever, we have no tracks, so what happens then?
I always called them 'drivewheels' as opposed to sprocket wheels. I guess too much time playing with model trains, though it is probably most accurate for T-34s which don't have sprocket wheels in the traditional sense of the word, instead relying on a series of roller bars within the wheel which mesh with the track's center guides. Things also get confusing with things like the BT-5 which had powered roadwheels and could run without tracks.
To break track on a modern tank, you need to release any track tension, apply track jacks (one to each side of the track, they look like large 'c-clamps' and are designed to keep the track in a loop when the connectors are gone), loosen up the bolts on the end connectors, pull them off, loosen up the bolts on the center guide, pull that off, then loosen the track jacks.
On T-34s (and M113s while I think of it), they use a single-pin connecting system on untensioned tracks, it's just a matter of hammering out the connector pin. Track tension is used to stop the loose track 'falling off' the wheels on corners. The teeth are there for the same reason. (Center guides)
To my knowledge, no German tank after the MkII used bogies. The interleaved tanks had torsion bars.
The missiles from the Russian tanks are usually beam-riders. (They ride a laser beam to the target). I'm not sure how the Shilleleaghs on the American tanks were guided, truth be told.
Tank speeds are limited by the power-weight ratio, gearing ratio, the friction of the track assemblys, and the ability of the whole thing to hold together as it drives. Theoretically, an Abrams can break 100mph. The track pretty much ripped itself apart when it did it though, so they never did it again. (Early model, stripped down for the speed run)
Tanks aren't going anywhere. They've had their doomsayers ever since the Israeli Armoured Corps met the Sagger Missile in 1973. They will always have a purpose.
As for the main role, modern tanks began to lose sight of their true roles during the cold war. They became more tank destroyers than tanks, but only in recent operations (read: Iraq) have some countries like the US figured out that the true role of a tank is multi-role, and generally infantry support. Hence the mad rush to put new ammunition into service.
NTM