get some kills by tower-diving and never get punished for that.
There are either two ways someone is tower-diving and not being punished:
A) someone is exploiting a timing window (some demigods have massive jumps in their capabilities compared to same-level demigods at some levels), combined with having both hp + mana flag so they are 1.15x a demigod anyway. This is typically known as "good play" and part of "good play" is knowing when someone else has an advantageous timing window and not falling into it. Even if they exploit that timing window, force a port, die but still manage to get a kill first, it can work out because then on the other side, they can 2v1, hard push for the tower, then one can port back to defend the opposite side.
no one supported the dying teammate for whatever reason - either good interrupts on the aggressor's teammates ("good play") or "poor play" on the victim's teammates (not having a port on hand or bad map awareness).
I suspect when someone is "tower diving" you're attempting to run away instead of fighting in the towers (or you overextended in the first place and you're being punished for it).
One has to be realistic and to adjust one's strategy
Demigod has extremely little strategy to adjust. There simply isn't enough depth to the skill builds and item selection. You do have to adjust "tactically" however. Such as focusing on levels over warscore and kills.
reactive DGs (Rook, Sed)
Umm... Rook is proactive. He's a lane pusher and he's the only demigod that can prepare for a major conflict 30 seconds before it happens. Second, I pretty sure Sedna gets punished in high towers? Although she does have 20 useful skills - more than a lot of other demigods - her basic bread and butter skills don't scale very well with stat inflation, and she can't deal with giants. The only skill points that do scale into artifact games are HW and Mag Presence.
She basically degrades into a buff-and-silence bot in the artifact stages of the game, and even those starts to slide off into irrelevance once people have multiple artifacts (Akshandor can crit-kill you before monks can heal you up, skills matter less once weapon damage starts going off the charts).
Really, out of all the demigods it's UB/Oak who benefits the most from high towers.
Maybe you don't typically play games that get to the artifact stages of the game but... I mean really? If you're playing High Towers and not even getting into artifacts then I have a hard time calling those 90% of games "skilled" - or at least "balanced".
There are some advantages to High Towers (I don't consider artificially prolonged games and higher fault tolerances advantages):
1- minion builds are extremely difficult to cheese with
2- ummmm... that's it? At least for things that don't directly derive from the two quoted exceptions above.