Listen to the Dive Season 1, Episode 17.
The whole mentality of subs is a threat to the starting role. And this creates an environment that does not create any synergy. Think of Faker and easyhoon 2015. Some say it worked out well. Sword and Shield? blahblah. Look at 2015 worlds. How many games did Easyhoon get to play at the finals? When stakes are high, you are going to put in one player and one player only.
Now how do you change the dynamics of the 5 man roster with subs? Well I proposed my original post. The issue I address is to attack the threat a sub poses to the main roster. Instead of "I am sitting out for a game", rather than "I am going to get advantage by getting more vision, tendencies, and therefore I can contribute XYZ to the team at XYZ time"
Again your comment on D-tier subs would get paid as much as people on stage. That is a cop out blanket statement, as salary is a huge topic to cover and would vary. "people on stage" do not get the same salary to begin with. I do think Subs should get comparable salaries to starters ---IF--- they add the similar value. In the current sub system it is hard to justify such value. And thus there are D-tier subs to fill the roles (tsm's regi @ mid lane). But by addressing the issue of changing the impact of subs (to the point of having subs is more advantageous and used every game/series), these D-tier will be replaced and filled with top talent.