Great retrospective and some fair and balanced postulating! For old people like me it's a good memory refresher. An interesting follow up might be the 5 most important moments in Type I's history (assuming Type I starts from January 1994).
in no particular order
*Removal of power level errata from Time Vault
As you pointed out, exacerbated by Tezz win conditions could now be condensed down to less than The Deck levels.
*The Creation of Planeswalkers
Paradigm shifting card type that in a sense obsoleted the big midrange creature end game. Jace and Tezz in particular being wincons and engines all in one card.
*Black Monday (June 20, 2008)
Rather than some of the surgical restrictions the DCI takes a machete to the stock 4x playsets found in many blue strategies. This is the last time they take such a howitzer to any format really.
*The bifurcation of Type I and II's B&R lists
A key change that would make future decisions far more sensible. This obviously had to happen at some point but when it did it was pretty earth shaking. Vise Age was dominating Type II but the Vise was holding Necro and the Deck somewhat in check in Type I.
Phyrexian Mana & Delve
I tossed these two together as 2 of the bigger design "mistakes" that not only mule fucked Type I but also made Legacy miserable for almost 2 years in aggregate. Phyrexian "Free" and Delve "Free" went together like peanut butter and jelly, with Dack and Token Spammers as the bread. IMO a few cards died for the sins of these design errors.
The printing of Force of Will
90% of the people playing type I now don't know what a world is like without this card or it's cousins. Maybe more than 90%. But it's printing fundamentally changed how type I was played. How decks were built. It forever changed the bluffing and going for it subgame in Magic.
Anyway, just some ideas for a top 5.