Not only would people often not play in one sitting, but many strategy gamers aren't above save-scumming either. Result, a large fraction of game hours on X-Com were on the easiest difficulty level.
You can plan all you like, ie save-scum. But you can't keep it. You can only make forward progress that counts, when you first disable your save-scum ability for a bit.
Of course, that only works for questions of probability, like hitting shots or not. It doesn't address that save-scumming also reveals information you are not supposed to have.
There's a middle ground for X-COM, called bronze-man-mode (in contrast to iron-man-mode). In bronze-man-mode you don't get to save-scum inside of missions. You can at most reload the whole mission to get another randomly generated one.
I used to play the original X-Com as as save-scummer when I was younger. I went back to replay a bit of the game via OpenXCom and stuck to a more 'mature' approach. The game was more fun that way.
I found myself making every move so calculated (and seeing it as a massive mistake whenever I failed at that) that it actually took away some of the atmosphere of the game for me. I had too much of a motivation to make the game less exciting and to make it as little about chance as possible, and in a sense it began to feel like work rather than an atmospheric story.
It might be a bit different in the original since IMO it seems much more difficult to calculate your moves and their potential outcomes as accurately. But that also makes it even more brutal in the original. (Or maybe I'm just worse at playing it, but that's how it seems to me.)
The first reboot is susceptible to overwatch crawl. And if playing in Iron Man mode that's what you (or me) might end up doing automatically.
They tried to fix that in the DLC and in XCom 2. With mixed success.
The original also had that problem: if you waited for long enough (35 rounds or something) the aliens would come rushing towards you.
That changed once you had access to psionics and the blaster as weapon. It became a large shoot 'em up afterwards (probably not at higher difficulties).