What you've describing is not so much a change in definition as a change in doctrine brought about by the results of battle and modification to the tanks themselves. Gun systems at the start of the war that were thought world beaters were proven impotent, armor was found easier to add to this tank over that one changes made and classifications adjusted. It happens not just in tanks but also naval shipping where today's destroyer is tomorrow's frigate.
Keeping with T55/54 we find a classic case of such. A Medium tank (evolutionary speaking most consider Medium Tanks as the direct ancestor to Main Battle tanks hence its not improper) at the time, second only to the Stalin Heavy Tank. But today T55 is looked on more like a light tank? Why? Because time when't by. The gun a 100mm cannon was surpassed by 115mm,120mm 125mm. Engines and horse power went up and armor became heavier and more refined. T55 was not able to keep up. Against Abrams in Iraq it was slaughtered. So the class of a tank has to be compared by era, yes. And era refers to its peers. Todays MBTS typically weigh between 40 and 80 tons using compound armor with a main gun between 120 and 125mm high velocity with a 1300hp or better power pack. If the tank falls short of these its not a MBT in the modern sense.
If it out performs these it changes the modern definitions and may force reevaluation by other nations of there Tanks pushing a new generation.
Furthermore the role of the tank is also a factor. A light tank in modern terms is more a catch all category for obsolete tanks with sub caliber cannons, tanks built on APC hulls that sacrifice armor for logistics. [see Marder medium tank], tanks that fit special niches. For example amphibious tanks a true MBT is a land fighter, a amphibious tank sacrifices armor for sea worthiness'[see PT76]. Or a airborne tank which sacrifices armor to allow it to be air transported in numbers aboard smaller transports[see M551 Sheridan].
Generally speaking in a gun dual a light tank cannot survive direct attack by a Modern Main Battle Tank. That is not to say that a light tank cannot take a main battle tank in proper conditions. If it has the gun system on its side it may be able to win. Most would have to attack the sides or rear. A few like the Thunderbolt Armored gun system block II a light tank that packed a 120mm Tank gun. From cover employing that cannon with first shot on target it can take most most Main battle tanks.
This leads up to look at the new emerging class of MBT in the true medium weight. The Japanese Type 10, Polish PL01 and BAECV90-120 are they really light tanks at all? They seem to be a attempt at moving from armor to stealth. Using reduced signature to allow them in theory to sneak up on a MBT and engage with a full power gun. They could indicate the next generation of MBT will be based around Hybrid power packs with smart skins and low radar cross sections.
I am not sure where this is going, yes, there are some circumstance that a light tank can take on an MBT, but that a rarity, and if that rarity justify the existence of light tanks is the question; sure if the country is rich enough.
Also, what you said about the T54/55 is true, it is a dated design, but it doesn't mean that it had not been upgraded for the modern battlefield with the official version being:T-55M6, 2A46M 125mm main gun, protection level to T80U level; and the new fire controls. So technically, the T54/55 did keep up, maybe falling on the short end, but it have a 125mm gun, armor around 1000 RHAe from the front. And your definition of a MBT will exclude the T72 with the 780 hp engine, or the Type 96 with the 780 hp engine.
And it is unfair to compare the gulf war T-55 vs Abrams which is not really purely tank to tank, but C4I2 with better tanks, vs C2I tanks. Would you claim that Abrams are bad tanks because the ISIS was able to easily take out a few of the iraqi M1 tanks?
The question of which is the direct ancestor of the MBT is debatable, there is no right or wrong, but:
Centurion family: Mk1-4 heavy cruiser <-> Mk1-4 universal tank -> Mk5-10 MBT
T44 family: T44 Medium -> T54/55 Medium/MBT -> T62 Medium/MBT ->T72 MBT -> T90 MBT
M26 family: M26 Heavy-> M46 Medium -> M47 Medium -> M48 Medium -> M60 -> MBT
Regarding performance, we also have to remember that some country categorize tanks by weight (like the USA pre cold war) and some by combat potency (like germany pre cold war) and some by some by surviability (like france/britan pre cold war).
in your definition an AMX13 with the 105mm L7 derivative, 40mm armor at 15 tonnes is a what tank? given that you you feel that the T55 with a 125mm gun, +400mm armor, at 35 tonnes is a light tank?
I am also not sure how you get low RCS on a tank, generally speaking, it is kind of moot as ground clutter makes the detection of tanks by radar hard already... and it is hard to reduce the heat signature of a 1000+hp engine; and 40-80 tonne of metal is not naturally found in nature..