Combustion engines are still the only viable option for long-range, large-scale heavier-than-air flight, for marine transport (on oceans, and most lakes, rivers, and canals), for many mobile power operations ranging from handheld tools to remote power generation, and for much overland heavy cargo transport (trucks, much rail).

Some of those can be electrified, but there are likely always to be exceptions in which that is not possible. Electrification is most viable where usage is heavy. Tracked vehicles electrify more easily than road-based ones (though yes, trolley busses are in fact A Thing). Canal traffic can be electrified through use of onshore "mules" (electrified traction), though passing and overtaking become concerns. River and lake traffic is less suited to this (again, possible in cases but not entirety).

High-latitude sites as in Siberia, Alaska, Northern Canada, and Antarctica cannot rely on solar power, and would require either a fuel-based or nuclear-based generating capacity. (McMurdo Station on the Antarctic coast had a nuclear plant, that didn't go so well.)

Mind that not all of these uses have a high demand for low-flammability fuels, which applies most specifically to aircraft. But all tend to rely strongly on fuel-based energy systems, and substituting for those is exceedingly challenging. The alternatives are effectively:

- Continue use of fossil-based hydrocarbon fuels.

- Find an alternative non-fossil hydrocarbon fuel analogue. There's been interesting work, and commercial industrial creation, of synfuels dating to the 1940s, though that was coal-to-oil conversion in Germany and South Africa. Generating fuels from CO2 sourced from seawater or the atmosphere has been researched since the 1970s at M.I.T. and the U.S. Naval Research Lab (USNRL), with technical proof but to date no commercial success. Google's X Labs attempted commercialisation under Project Foghorn, but failed on economics: <https://x.company/projects/foghorn/>

(I've reasons to argue that it's the economics of fossil fuels, not synfuels, which is principally to blame here.)

(Biofuels are often suggested. The problem here is that the net likely capacity is at best a small fraction of present fossil fuel usage. We might feed 5--10%, with the upper bound being highly optimistic, but we're not going to replace 100% of present fossil fuel usage, let alone the future growth required to bring under-developed regions of the world to even a small fraction of industrialised nations' consumption.

- Substitute non-combustion energy plants. To date that's nuclear, which ... has its own challenges. For larger fixed-site locations, that's possibly viable, though there are numerous cautionary tales to heed. For mobile applications such as rail or marine transport, the compounded risks of already probable accidents make this highly unattractive.

- Substitute renewables. The prospect of resuming sailing ships for international sea cargo is floated, though cost and scale of ships would likely be impacted (rising and falling, respectively) tremendously.

- Drastically curtail or cease such activities. One of the realities of economics is that as expressed costs change, so too do activities in which those costs are incurred. The fossil fuel age has made transportation unbelievably inexpensive compared to pre-industrial times. It's possible that we'll see considerable back-sliding on both personal and cargo transportation as fuel costs rise. This will of course profoundly re-shape the world, in much the way that cheap transportation re-shaped in our recent past.