At the rate of our technological improvement, any journey taking a significant length of time (let's say > 100 years) could be surpassed while in progress for a faster cheaper mission. Actually this is probably true for missions as short as 15 to 20 years. Better to wait for technologies on the cusp of incredible improvements like nanomechanical systems to mature before designing extremely deep space missions. NASA should do no more than very light funding to understand enabling technologies for far future missions.

There are some future thinkers thinking about such things as massive solar sails carrying extremely light payloads which might provide a relative cheap way to get a reasonable acceleration beyond the solar system.

One of the most practical constraints for whether a mssion is feasible is money. if we set 5 billion (today's dollars) as an upper limit for mission cost, then anybody can come up with any mission to compete against others at cost. When extraterrestial probes are doable at less than 5 billion they can compete with the other ideas. Cost is a great guide for whether something is truly practical. And it makes it simple to understand when and if enabling technolgies for a mission truly exist. I do think reasanably priced "prizes" can be an effective way to incentivize the development of enabling technologies.