The 'Mercy Main' issue

Basic outline:

Primary Proof
GIVEN:
   a) Mercy is the only consistently playable Heal-Support;
   b) Every match needs some manner of sustain;
   c) Players will often be 'trapped' in a role;
THEN:
   Mercy Mains are a symptom of more than just the SR rewards for 'maining' a hero

Proof a)
GIVEN:
   i)There are only 4 healers (Mercy, Ana, Lucio, Zenyatta);
   ii) Only Mercy and Ana can be played without an additional support, and Lucio/Zen requires a high level of team coordination resulting in a low TTK;
   iii)Ana requires a higher skill level, and is unsuited to the current meta;
THEN:
	a)

Proof b)
GIVEN:
   i) Damage output is generally high and 'naked' TTK is low;
   ii) The game is designed with healer classes to enable longer fights despite this;
   iii) 'outkilling' the enemy reliably requires a level of coordination not present at most SRs/with most matchmaking-made teams;
THEN:
   b)

Proof c)
GIVEN:
   i) Players gravitate towards certain play styles based on what they find rewarding;
   ii) Many players find the effect they have on the game as DPS more rewarding than that of a healer;
   iii) Other players find winning the game more rewarding than getting lots of kills but losing;
   iv) Players will encourage people to play something based on previous heroes played;
   v) In general, DPS players are loath to play tanks or supports and will instalock DPS;
THEN:
   c)