It's been quite a long time since the first cryptocurrency with the "Mimblewimble" privacy technique was released. My first days of using GRIN were quite troublesome due to its added complexity. But it's a great coin to use for privacy-oriented transactions once you get the hang of it. Somehow, GRIN became a forgotten cryptocurrency as all of the attention shifted back to Monero. Was this a result of poor marketing/promotion efforts? Or a lack of user experience?
I'm wondering if this coin still has a future, especially when privacy on crypto has been frowned upon by mainstream governments. Do you think it can be good long-term investment like Monero? Or is it a complete bust? Your input would be greatly appreciated. Thank you very much.

Depending what you mean by thing? Is it interesting to majority or trendy? No. But İgno (founder) suddenly leaving grin was probably end of grin, as even with new developers, that kind of move make people lose trust and interest. Also i am not sure if there was any need for competition as we had monero already. Especially when, like you said that it had "added complexity".
Actually i doubt that there's any privacy centered coin that is doing fine in this bull market. Regulations from FATF made sure that they won't be in any big exchanges, and even cashing them out in any way gets more difficult as time goes on and these new regulations can be monitored more efficiently.
Here's a reddit thread from 2 years ago about it being dead:
https://www.reddit.com/r/grincoin/comments/rhhhlk/so_is_grin_dead_or_what/