Well, here's my opinion.
Yeah, "Save the Ralari" is fun but it's a non-critical mission. If you lose, you go to Rostov, which is a very cool system, and if you win at Rostov, you go back to victory path on Venice. So, if you lose the Ralari mission, it's even better because you get 3 cool missions with Iceman.
About the other mission, it's just a matter of politics. Some games are made artificially harder because they limit the save game process. Take the original "Dark Forces", for example. A mission could take like 40m, but if you lost after 39m of work you'd still have to replay everything. That mission uses the same trick. It's not that difficult, you just have to replay it a few times... What's hard is to repeat 10m of so-so gameplay so you can get to the hard part.
WC games had a good balance. You have somewhat short missions, and can't save during them. But each time you land, you get a chance to save. So you never have to replay too much in order to get to the point where you are having trouble.
It's not about not making the mission too hard, but to impose a lot of repetition in order to get to the hard part. That mission 3 has 10 minutes of repetitive, moderate combat in order to get to the difficult part. So each time the player loses, it must “autopilot” for 10 minutes until he have another chance. It gets boring very quickly, even for the hardcore gamer. A casual might simply give up after playing those missions. People play game to have fun, you know, not to suffer.
And I do think that good games must be fun. Games that aren't fun generally are bad games. The Ralari mission was fun because you could lose it without any great consequence. Also, it didn’t take much effort to get to the Ralari… In fact, you could even bypass the Salthi on the Asteroid Field and dash for it. If the Ralari mission was necessary to win WC1, people would hate it.
For another example, take a look at X-Wing. It was so difficult people would cheat simply give up. LucasArts ended up making the missions easier for the re-release version. It had a very bad game design. Missions were too hard, and if you didn't win a mission, you couldn't advance on that tour of duty. Also, missions were too hard sometimes. And if your character died, you'd lose all your stats or you'd have to start from scratch.
So for the “harder is better” guys, why not just delete the savegames and start from mission 1 when you lose at any point? That must sound great for you!