In my experience, the last 20-25 stays (between 2016 and now) in both Marriott and SPG, I was offered an upgrade about 80% of the time. Other times, I kindly and respectfully asked the rep if there is any upgrade available today, and there always has been for me. My advice is, it doesn't hurt to ask, but don't beg or fight either. Also, the earlier in the day you check-in (between 3pm to 6pm), the better chance you have at an upgrade. I have never been denied an upgrade-- except at Hilton properties as a Hilton Gold.

I am a Marriott and SPG Gold. I book a basic 1 king room. This always gets upgraded to the usual 1 bedroom suite with the preferred view. Rarely if sold out I get a corner room or junior suite. At one point I was upgraded to a bi-level suite.

My stays mostly happen on Fridays/Saturdays/Holidays. I also always request 4pm late checkout. And these take place in major North American cities.