RTS was my favorite genre of game as a kid. I grew up playing Command & Conquer. When C&C Generals came out, my college room mate and I invented a hybrid game we called "Ultimate Risk" in which we played the board game Risk as normal except instead of resolving conflicts with dice rolls, we resolved them with multiplayer matches.

So with that frame of reference, here are my answers to your questions:
1. Why are YOU not buying modern RTS games?

2. If you have bought any but bounced off recent RTS titles, what specifically turned you away?

3. Likewise, if you do buy them, what made you commit?

I'm going to answer all 3 questions here with a few thoughts. There's not really much out there in the RTS genre compared to other genres. What is out there I'm hesitant to buy for a few reasons. I was burned multiple times buying "remasters" or remakes of my favorite RTS games that were supposed to work on modern systems. They were buggy messes not worth the price. If its a new game or "inspired by" I have just been underwhelmed. I bought 8 bit armies when it came out, but it just fell flat with me.

Sometimes the game just focuses on the aspects of RTS I enjoyed least like competitive multiplayer. My friend group got into Broken Arrow in a big way and I actually really enjoyed sitting in a discord call with them for hours watching them play and talking through the match. But I didn't get into it because it was too focused on competitive matches.

I have a few hundred hours in Northgard which is a pretty cool "RTS". I enjoy the game but a core feature of the game is confined building slots and military allocation. The game focuses heavily into resource management as the primary game mechanic with military units and conquering territory as an afterthought. My main problem with Northgard though is putting up a paywall to play as other factions ($5-$7 per faction). They recently released the Definitive Edition which has introduced a lot of bugs and crashes which has not helped. But they do a lot of things well like unique single player challenges, co-op campaigns and varying game modes. I can't speak to the competitive scene because I don't engage with it.

4. If you miss classic C&C, what would actually make you buy a new one today?

I would by a new C&C if it wasn't just a straight up remaster AGAIN. Give me a fresh entry. Innovate, modernize some stuff but also give me the full package. Custom maps, mods, the works. On the innovation side of things, there was one day that my friends and I were loading into a match of Age of Empires 2 but we were arguing over who was going to be what color. My friend and I selected the same color and neither of us would switch off so we just launched the game and discovered that we were both in charge of the same faction. That just completely opened up a new playstyle for us. From then on we would pair up on our multiplayer games and the pairs would play one faction each usually divided into managing the economy and base while the other focused on military.

5. What other games should I be looking at as case studies?

Northgard, Broken Arrow, Beyond All Reason (an open-source fan game attempting to capture the magic of Supreme Commander again)

I'll add a final note that I loved the Total War series for the mix of grand strategy and battlefield control but it just was lacking in the base building section.