https://www.si.com/mlb/diamondbacks/analysis/starters-workload-stop-blaming-analytics-for-122-year-trend
In fact ever since 1901 the percentage of innings pitched by starting pitchers has continually decreased from previous years and decades. There is some ebb and flow of course, but the 122 year trend is undeniable, and did not just start since the popularization of modern analytics over the last 20 years.
The average rate of decrease for the entire 122 seasons is 0.3%. Notably however there are several points in history where that decrease was accelerated for a seven or eight year stretch.
The article is a quick and interesting read. It's free, too! There's a mention of the impact of the live ball era, integration, analytics and the impacts that these have had on the overall quality of play that has reduced the ability of starters to simply pitch longer into games. It's the natural evolution of the game that has occurred for 122 years and should continue. Perhaps there is a threshold where innings will flatten out, but MLB doesn't seem to be there yet.