**SPOILERS**
Two key points to make in response to the original poster. As a point of background, I'm the DM of a 5e campaign, and we completed the scenario in discussion, and have been an alpha playtester throughout. I also played 1e, 2e, 3e, 4e, extensively.
First point - your primary concern is about high-end game design, and the addition of immunity to 1st-6th level spells. True! Resistance to all would have played much more elegantly, with the notable exception of my second point. It's worth considering in the future. 3e's epic design played this way, and on the whole, it wasn't fun. Agreed. 4e high-level fixed this flaw, and it should be considered even for big bosses. At a minimum to make them "last" longer - give them triggered or lair effects to maintain a heaping helping of high level hit points. There's always more than one way to design through the obstacle, and your point is this solution isn't very fun for wizards. I agree with your point.
Second point - Prism mentioned it, but it should be reinforced - my reading of the last encounter was ENTIRELY different than what you described. As I ran it, you were NOT supposed to fight Tiamat, but PREVENT her from being summoned. There were ARMIES, including "as many dragons as you want", purple-robed cultists, giants, and even dragon leaders in ten different temples throughout the massive Well of Dragons. You had only 10 rounds to outsmart the armies, guardians, defeat the cultists, stop the summoners, and ultimately confront and defeat Severin. For us, this was ultimately a raucous slogging climb, resources completely spent, finally reaching Seveerin, their deadly leader, with one chance to kill him before he unleashed his full wrath on one PC at a time, eliminating them. I found it a fantastic captsone fight to the end of our campaign. All the villians I described were all susceptible to all forms of magic, and abilities from the characters, and it took everything they could muster to survive the death march. Also, our highest level character was 14, with average party being around 12. It sounds like you had a decided advantage on our PC's.