I have a few points:
1) Teleport does not inherently ruin a game
2) We don't know how much any of these spells cost... you can balance spells by changing their cost
3) every 4x game i have played has had overpowered technologies/spells.
4) Not all strategies are viable, in 4x games there are usually a few that rise to the top.
5) Balance is much more important for competitive multiplayer games than predominantly single player experiences.
1) In my experience it always becomes the main focus of the game (Heroes of M&M, Age of Wonders I, and any other TBS game I have ever played... meaning almost all TBS games ever made plus many fantasy/scifi board games) - You may say you like that, of course, but such a overwhelming option takes over the whole game, thus limiting its strategic options, and therefore its lifespan. To me that means that it does "inherently" ruin a strategic game.
2) Cost to me is not an issue, since teleport=victory, therefore any price is a price I would be willing to spend.
3) True, but it's their weakness and when games get to that point victory is just a matter of turns, fantasy games turn into sci-fi games with their strategic approach pretty much dead (strategy is mostly movement, let's not forget that!)
4) ??? Not sure what you mean by that one. In Age of Wonders shadow magic there are tons of different strategies, very different among them, that may bring you to victory. (defensive examples: elven cities invisible in woods, an aura of darkness that covers your territory and limits visibility, all sort of defensive spells active in your territory... offensive example: units that are invisible in woods to evade the enemy and approach unseen their cities... Flying machines... All that would become much less relevant with the use of teleport)
5) Well, that's a matter of taste. Personally I don't like when the AI is unable to do what I do or counter my strategy, but I am quite an experienced player, newbies may like a little advantage. Still an unbalanced game, as the one you seem to suggest we are about to get, won't have a very long lifespan.