"Once fusion is cracked" is almost on par with "once perpetual motion machines are cracked." It presupposes that fusion being "cracked" is a reasonable thing to expect.
Oof. One need only to peruse your comment history to see where this sentiment is coming from, but even that is no excuse for that obtuse argument.
"Perpetual motion," on the one hand, is quackery outlawed by straightforward thermodynamics, breaking the rules of which allows assertions like "chair seats and door handles should be spontaneously heating up to incandescence essentially at random." Fusion, on the other hand, is a technical problem.
Fusion researchers at MIT, ITER, and other institutions at the top tiers of academia across the globe are not a cabal of greedy grant-dependent charlatans bamboozling their way through careers in bad faith, deliberately ignoring the lone rational voice of Lidsky and his Johnny-Come-Lately-The-Baptist on HN, u/pfdietz. It's perfectly rational (Sane, even!) to cast aside internet naysayers (no matter how zealous) in favor of deferring to, you know, actual experts.
In general, you don't want to ask a person in field X if X needs funding. You might ask them what's the best way of spending money in X, but even then you better phrase the question carefully to avoid conflict of interest.
Note that DT fusion is a thermal power technology. It makes heat that drives a thermal cycle. It turns out that all externally heated thermal cycles have become uncompetitive for power generation, just because of the cost of the non-reactor/non-boiler components. This includes fission, solar thermal, geothermal, and coal. So even if a DT fusion reactor is delivered FOR FREE from the fusion reactor fairy, it will not be competitive. Expecting capital equipment to have zero or negative cost is a good economic analogue for a perpetual motion machine.
So, what's the alternative? It's going to have to be something using advanced fuels and direct conversion. That means all the efforts with tokamaks are ruled out (they cannot work with advanced fuels and have no place for adding putative direct conversion.) And what advanced fuels are we talking about? D-3He would require space mining of the outer planets (the much-discussed lunar resource, even if it could be mined at 10 ppb in the regolith, would only last centuries before being exhausted.) And H-11B is extremely difficult, because the energy out is only about 10x the kinetic energy of the particles, so very little loss can be tolerated. In particular, systems in thermal equilibrium will have a very difficult time reaching breakeven, due to large photon losses.
Another showstopper is the absolutely terrible power density of fusion reactors. This follows from general principles (geometry and the square cube law). If you look at all existing fusion reactor proposals, ask what their thermal power/volume is. You will discover they are terrible compared to fission reactors (ITER 400x worse, ARC 40x worse). So ask yourself: why is this going to be cheaper than fission reactors? The fusion reactors are both larger, more complex, and made of much more sophisticated materials. And fission has already lost the economic race.
The larger more complex reactors are really bad news for reliability and maintainability. Estimates for the fraction of time a fusion reactor will be up, given current estimates of MTBF and MTTR, are just a few percent. This is inadequate even for an experimental reactor. There is not much budget space for operating costs from a fusion reactor before just that becomes prohibitive.
From a hard nosed engineering point of view, fusion is just ridiculous. It piles complexity upon complexity in a large machine that will be too radioactive for hands on maintenance. How could you think this would ever make sense?
I think you meant to say "I hope we develop fusion so we can leave the solar system".
Perpetual motion _engines_ violate law of energy conservation, because perpetual source of energy can create infinite amount of energy, thus they are impossible, like perpetual source of water, or perpetual source of anything.