In my view the programs (airline and hotel) are evolving entities, and you need to keep looking for the next best deal (of which there are fewer and fewer).
40 years ago I was a Sheraton customer. Can't remember the level. I was in an upgrade suite (a real suite, with a view of the Mississippi) in the Sheraton New Orleans during Mardi Gras.
Then about 35 years ago, I was one of the most frequent travelers in the US. (500k BIS, all domestic). I got a postcard from Marriott which said that if I would make just 2 stays with them, to try them out, I would get a free night. I used the free night for the Prince de Galles in Paris. When I arrived, they didn't show it as a reward stay. Luckily I had a fax (remember those?) confirmation. They upgrade us to a larger room, and sent flowers and champagne which were worth more than I paid for any other room in Europe. So I stayed with Marriott for about 20 years, getting phenomenal upgrades (my stays are 1 or 2 nights) and amazing personalized service (only required maybe once every 3 years). I got one of the first Black Cards (FD clerks would ask to see it, because they hadn't seen one before).
Then about 15 years ago, I noticed that Marriott had changed. I wasn't getting many upgrades. So, with some reluctance, I changed over to SPG. I was pleased because I got more upgrades and then they added breakfast. Of course, their website IT was among the worst of any company I have ever seen (you can find my posts on FT saying just that). But it wasn't a reason to leave.
Then, about 3 years ago, I noticed that SPG wasn't quite the same. So I began to book my hotels with a greater emphasis on my personal convenience. Stlll lots of SPG, always lots of Marriott (I was LT Plat, so I still used them, just not as much as SPG), and others.
I am NOT currently a frequent flyer (i.e - more than 300k BIS/year). I am not even a busy flyer (i.e. - more than 100k BIS/year). So my current opinion is based on judgment and history, not as many specific interactions. But what I see is the unfortunate decline into "the computer won't let me" that is affecting MANY businesses (travel and otherwise).
P.S. - My current favorite program is the one with Omni hotels. Why? Because I am a frequent visitor to one of their properties where they treat me VERY well. But repeat business means a lot. I used to be on the "50 top customers" list at the Westn Bonaventure and I was treated very well.