Just to be clear, the Titan "boat" was really a buoy. It would have dropped into a lake and drifted. That still would have been amazing (imagine video from a lake on another world!), but the science would have been limited by the cost of dealing with the environment and the tight budget of the Discovery program.
The biggest issue, however, was the problematic development of the needed power supply[1].
There is another option for these missions, of course. A nominal increase in NASA funding to increase the number of missions we can fly.
Even these "cheap" missions are a half billion each, so it's a little more than nominal. Choices have to be made, even if NASA's budget was doubled.
TiME was always a longshot proprosal, but I'd personally rather see the lower budget Discovery Program missions doing riskier stuff. And while I love a good Mars mission, Mars gets all the marquee missions anyway. It's kind of a shame to see it eat the resources of the lesser missions too.
The biggest issue, however, was the problematic development of the needed power supply[1].
There is another option for these missions, of course. A nominal increase in NASA funding to increase the number of missions we can fly.
[1] https://en.wikipedia.org/wiki/Advanced_Stirling_radioisotope...