Actually the M60 had more in common with the M103 and M48 than the original M26. the M46 and 47 tanks are direct decendants of that tank while the M48, despite sharing some components with the earlier designs and looking superficially similar to one another, they are all quite different. The M60 in fact was a new design that also shared some of the components from the M103, as it was meant to be its replacement.
More to the point, I don't recall the DEVs stating anywhere that the "MBT" title blackballs any vehicle from entering the game. Which for most people includes the T-55, even if you don't consider it as such.
Let me answer your statements one at a time here:
Actually it would matter, you have to look at the historical role the MBT plays in armor development. These vehicles were supposed to not only replace the Heavy tank, they were also meant to be direct replacement for most armies medium tanks as well; essentially a 'one-size-ftis-all' vehicle. The speed and maneuverability of a medium tank with the weight and probably 90% of the armor of heavy tanks. Add to that advanced armaments that while not physically larger than their heavy counterparts, were still powerful enough to kill nearly anything they hit and were supremely accurate. (I should note here that accuracy in this game when refering to tanks of this period is relative as the physics engine in WOT is probably incapable of modeling a Fire Control System.) Now you might say "But Trophy_Wench, what about the E-100 or the Maus or the T95 GMC etc. Those heavy tanks still dominate the game and even the capabilities of you're so called MBT would have just as hard a time killing them as any other medium or heavy!" Well you'd be right in thinking that but understand that those particular examples are, for all intended purposes- freaks. Superheavy tanks that would not normally have any basis in reality but here in the vacuum of WOT they work just fine.
Secondly, you're right. At no point have the devs said that MBTs have no place in WOT but the reluctance of them to even entertain the idea of a tier 10 medium tank is a sign that they are not intrested in armor developments beyond the 1950s. Now as for the T54 and Type 59, which are essentially one in the same, you have to look at the context in which the T54 was developed. It was designed, developed and deployed in an age where the term and concept of 'Main Battle Tank' had not been thought of yet. There were still clearly defined roles for Heavy, Medium and Light tanks on the battlefield. However the designs of these new "medium" tanks were so good that they, by default became the standard all purpose tank in their respective armies. (The T54/55 and Centurion are the most prime examples.) In doing so, they relegated the Heavy and Light tank designs to fulfill more niche roles until the development of a true MBT in the early 1960s squeezed the Heavies out of the picture once and for all. So technically, it is both correct and acceptable to have these tanks in this game because it is only the modern context that we continue to refer to these vehicles as Main Battle Tanks.