Some things that might de-skew my evaluation of exciting new projects:

• If the project is new, it probably looks shinier than it is. I should wait a while or try to correct for this before evaluating.
• I should take into account my record of defecting halfway thru a project to discount the utility of the new project.

If you assume that your future self will make a rational decision about whether to defect, this is unnecessary.

• The latter half of a project is relatively untouched territory, full of valuable new experiences. I will have to work thru the first half of the new project to get to this, but I am already at the threshold on the current project.
• Maybe there's more?

Yes, remember the cost of getting the new project up to the point where the current project is right now. The sunk cost fallacy says you should ignore the "sunk cost" of work on the project you've completed so far; however, you should not ignore the "replacement cost" of that work.