I've also been at this for 20+ years and on both sides of the equation, and have seen a few patterns.
1. When unemployment is high, we get a lot of new shoppers entering the marketplace.
2. When earning vs. cost of living ranges lower, we get a lot of new shoppers entering the marketplace
The difference I noticed when working as an editor is that the cost of living issue during an employment boom nets better and more educated shoppers. They are usually employed elsewhere and looking for ways to cut down on household expenses. During times of heavy unemployment, new shoppers are looking more towards the fees and tend to drive pay rates down in competition for work.
The constant seems to be that once lowered, fees rarely, if ever, go back up. Most fees are equivalent to or lower than when I started in 2001, so basically all are lower when taking into account the average for salary increases since then.
Reimbursements have also taken a serious nosedive in the past 20 years. There was rarely an issue getting dinner assignments to come in under budget when I started, and purchase/return shops were rare. Most retail shops netted me product back then.
I have all but given up hope for any return to the way things used to be. So much would need to happen to bring us back to a place where MSCs had to once again compete to get qualified shoppers, and clients have always shown little interest in remunerating shoppers adequately. This is not neccisarily the death of the industry, but will probably result in the death of my interest in it. As it is, I only consider shops from a small handful of companies these days, when I was once active with over 100.
That said, we also live in a world where clients can now simply go to social media to see photos of their food presentations, and a majority of the younger generation feel like they are all restaurant reviewers, so there are new markets to get into if $$ and free food are what you are looking for. My step-son manages a pop-up restaurant and pays an influencer $600 + all the food they want every few weeks to drive business. Rise to the top of THAt game and it puts my long ago best year of $14k income and $50k reimbursement to shame....