It's naive to say something absolute like that. It's possible to do everything light class can do with medium, if not better. Don't take everything you see as the prime example of how to play this game. Just because the majority are using light in combination with one of the new cores, does not mean it's right or that heavy is suddenly obsolete.
I'm not being naive when I say that, it's a fact. I say it from experience, from both playing and observing the game and the changes done to it over the years. There has and always been only
one advantage for the heavier classes in this game, and that is extra points in health, but nothing more. Outrider, on the other hand carried 3 class advantages including greater acceleration, greater maneuverability, and most importantly, smaller hitbox(Did you know that the Raider and the Sentinel share the same hitbox? Meaning in theory, a Raider is just as easy to hit as a Sentinel. It's funny cause Raider was supposed to be
medium armour, yet he's as big as a Sentinel.).
Back then, when the game was "simpler", without the grenades, cores and some of the weapons we see today, classes were quite close to how they would perform compared to each other. With the addition of Boost grenade, new possibilities opened up for Outrider. Raider only got a slight increase in speed with the boost as it's "heavier", whereas Sentinel acquired nothing new.
Now add the newly introduced cores like Shield and boost, and now movement-wise the Outrider's ability to get from point A to point B has gone up tremendously, whereas Raider gets a smaller boost from the two cores(did you know that a Raider takes 21% to boost from 0-80 with the Boost core whereas an Outrider also takes 21% but boosts from 0-100? Why should Raider take the same amount of damage but get less out of it?). Sentinel, again, gets nothing out of it. Don't include the Shield core's advantage of damage reduction here, it's a trait shared by all three classes.
Anyway, my point is, with every new update if the Raider and the Sentinel gets say 1 more thing to do, Outrider gets 2. The same old "they got more armour" reasoning doesn't work anymore. The armour people always speak of is nothing but an added 25 points of health in return for all the other disadvantages I mentioned above. So if you think a Raider can do everything an Outrider can, or that they can do something even better, you're the naive one Xtreme.