It's my opinion that you should never base a purchase on a review. I feel reviews are intended to share your personal experience and opinions on media, and compare and contrast your experience with others, not to sell said media. Obviously the current gaming review landscape seems to disagree with that philosophy.
I agree watching let's plays/gameplay is a good way to gauge your interest, but with these sorts of games I feel like any and all spoilers defeat the purpose almost.
I agree fully with your opinion on reviews! Also, I agree with your logic on the playthroughs as well as it's true that spoilers shall be spoiled if done so haha I guess if it's for people that are very practical with where their money goes, most may take the risk of a spoiler here or there. Though, for me, with games like LiS, they are usually instabuys for me for the most part at this point

The only time I watched a playthrough of LiS was of episode 1 after when I first came across LiS in a copy of GameInformer and I felt that game was going to be good, but I had to see for myself since funds were very much low at the time despite how cheap the game was. It in the end, had me captivated and I was like "Ok, sold!" lol
Oddly enough, even though I saw some spoilers that may have been deal breakers for others, witnessing it in the real time for me personally by playing through it felt like a totally different experience. It was as if, even though I knew what was going to happen, just seeing it and hearing it while overall immersing myself in the game through my eyes by playing it made it feel like a whole different story. I know that sounds weird aha It's hard to describe without it sounding odd
I usually just tend to "feel it out" during a trailer and can usually pinpoint if I will enjoy it or not ☺ If I get goosebumps, then that's usually a good sign lol Sometimes, you never really know if you will truly enjoy something until you try it. For others? They simply know if they like it or not from the get go haha!
In the end, you will know what's right for you

Maybe one day you will try it! Or not! Either way, the magic that is LiS is something that anybody can talk about no matter what game they have played of the series that's for sure! ♥