Playerbase percentages don't tell the full story alone. There could just be more good players now.
What we need to see is statistics like cs/m, gold/m, vision score/m, average KDA, exp/m. We'd need to compare these year by year. We'd also need to adjust for gold inflation because everyone has higher gold per min now than in 2013 but for 2 reasons, 1.) There is more gold to get with platings and such, 2.) The playerbase's average skill has improved.
If we could get those stats for each year, compare their rate of change with the rate of masters tier playerbase growth year by year. If the player count in master is growing faster than the rate the statistics are growing then there would be an indication of elo inflation.