Alternate techs...maybe...
Alternate tech paths are fun to consider, but there are fewer viable alternates IRL than often seems to be the case IME when you have less info on the trade-offs between the actual systems and the alternates.
In the cases of the specific technologies you mention:
External combustion engine--only real advantage is marginal, the ability to choose your fuel. Even with a lot of development by top engineering firms they're significantky less efficient than IC engines. Developing high horsepower engines with any usable degree of efficiency is future tech at this point, whereas scaling up IC engines is far less difficult. Steam is potentially viable, but unless some external factor favors it over IC, IC wins. Also, it's worth mentioning that diesel _is_ an alternate IC technology.
Steam pressure artillery--"Hang on General! We'll have the accumulators ready for the next salvo in twenty minutes or so!"
Ornithopters--were given a fair shake several times over. They're still future tech at this point. A bird's control systems are far more sophisticated than an aircraft's, or than what we can build at present, and an ornithopter needs not only that, but a way of tying the flight crew in. It may not be too far off, but it's not likely to be a general technology any time soon.
OTOH, there are a lot of "alternate" technologies actually in use all over, either unrecognised or filling a likely niche where the dominant technology is not as good a fit. This is where new dominant technologies often arrive from. Just as steam displaced early hot air engines then internal combustion displaced steam, then diesel took its place alongside conventional IC engines. Steam is still waiting in the wings, and who knows, maybe some sort of Stirling will come back some day, though they've got a
lot of practical problems to solve (speaking from my experience with modern engines of the type.)
I sometimes wonder what would have happened if some techs hadn't come about, and had been skipped entirely. For example, electronic tubes of the diode/triode/tetrode variety. It would have been easy to miss. One well-placed streetcar could have done it. The concept of electronic semiconductors was already around in physics. Would they have come earlier if there had been no tubes? And would planar forms like those of today have dominated? What if tubes had arrived as an amazing new high-power electronic technology in a world with a mature semiconductor technology?
I guess this i just another way of looking at alternate techs--more as just a re-ordering of dominant technologies. *shrug* You can probably tell that I'm as interested in thinking about alternates as any gearhead SF guy.

Like, if we'd actually finished any of the three Shuttle "replacements" I worked on.
