You're right, I do agree that Dominion's end is the final milestone to close the big era of League's rise to fame, aka LoL's puberty. For a bit of nostalgia, I started during Thresh's release, at the beginning of S3. Back then I had no clue about League's story, balance or meta, but now that I look back to it I can easily evaluate the environment at the time.
First off, new champions were made with little regard for the impact their release might have had. Champions like Thresh and Aatrox were introduced regardless of how negative their presence was. The pro scene was still being shaped up, with the Korean threat still looming just beyond the horizon. As for Dominion, I used to consider it a fringe option for the more experienced future me, yet the more invested I was becoming in LoL, the less attention I paid to it. Now I am not even surprised to see it gone. Lastly, today's playerbase is so entitled that they consider every change undocumented ahead a "lack of communication". Some of the changes in the past would be considered "surprise reworks" by today's standards (e.g Karma).
I am happy with the way League has gone since then. It matured in a lot of areas, although it still has its flaws like every game does. For one, LoL has changed the current game industry a lot, and by the end of this decade I will be surprised to not see it in a hypothetical "Top 10 list of the most impactful games of the 2010's".