There is plenty of content without timers. The M+ timer is what makes M+ exciting and what pushes you to improve at the game. It is also makes the required time for a dungeon run more predictable, i.e. you know you won’t be in there for two hours, people won’t afk for a smoke, check on kids, make some food, take a bio break, etc. in a clearly defined block of time. You may go over by a few minutes, but almost all runs are +/- 10 minutes.
The timer does not make M+ inaccessible to “casuals”. There is no connection between the two. You can be casual, e.g. play a few hours a weeks or not have competitive ambitions and still be able to do more challenging content (and want to do it). Likewise, you can play the game for eight+ hours a day and still not be interested in challenging content.
Everyone finds their ceiling in M+. For me right now it’s somewhere around 22, for some of my friends it is at 26 this season, other friends don’t venture beyond 18s, yet others are happy with 11. It’s a very scalable system and the timer helps that players stay in the content that matches their current skill, experience, and interest level. If you remove the timer, more people would probably have a miserable time because they’d start doing content that they lack experience or skill for (cf. Cataclysm heroics before they were heavily nerfed).
The reward is the sense of accomplishment when you complete a key in time that you had not previously managed to finish within the timer. If that’s not for you, you can just ignore the timer. Like you said, you still get loot.