I have often thought about this problem in the Trek universe myself. He's absolutely correct. It should never be easy to time travel, not in that science fiction franchise, or any other for that matter.
Slingshotting around the sun was the most ridiculous method, but if you can ignore that one all the other ways it was done were quite difficult and not something a standard starship could attempt every day. For example, in Star Trek: First Contact they rely on Borg technology that they did not possess (although they did gloss over quite a bit how they returned).
But the way I see it, there are really three solutions to this problem that maybe the writers in Trek can tackle after they pop the corks on the champagne as they celebrate the opening of the 2009 Star Trek movie:
- An episode of a future Trek series or movie could depict a starship trying one of those methods on purpose and it goes horribly, horribly wrong, and everybody on the ship dies or it creates a massive rip in the space-time continuum. This would show that it's extremely risky and why people aren't trying to time travel willy nilly.
- The writers slingshot around the sun, or borrow Matt Frewer's time machine or something, and go back into time and prevent themselves from ever writing those episodes or tell themselves to come up with a more plausible and risky time travel method
- You realize that this is just fiction and you go try to solve some real world problem like Global Warming or whatever.