I'm not sure about this assertion. Hasbro doesn't put out new editions of Monopoly or Life with changes to the core rules, they just dress them up in a new skin every once in a while. They don't spend money on development, just on marketing and sales. D&D could follow that path, needing only enough sales to new players to cover those costs. As long as they maintained a healthy market share, there would be no reason to create a new edition.
Market share isn't enough, it's about profit and profit margin. If D&D doesn't create enough profit at a large enough margin, then something will need to be changed. This happened in 4E, which did fairly well initially, but failed to meet the unrealistic expectations set by either Hasbro or WotC. TSR had a huge amount of the market share during 2E, but still went bankrupt and had to sell to WotC. These are the unfortunate realities of corporate business.
If sales really drop, it wouldn't mean putting out a new edition, it would mean dropping supplement creation down to 1 every year or three. If it's popularity picked back up, they could go back to three or four per year.
Cutting the total profit to increase the margin isn't enough. While profit margin is important, WotC will need D&D to provide a certain amount of overall profit. If the brand isn't producing a lot of profit, even at a high margin, the opportunity cost will make it not viable. I have heard the concept referred to a "throughput," but I don't believe that is the actual economic term.
If Hasbro wants D&D to be a game played through generations like Monopoly, Risk or Life, they can't keep reinventing it every 5-10 years. And the bottom line is the minor flaws in the current system are vastly outweighed by the ease of use of the system and it's accessibility to casual gamers, who are driving sales year after year after being introduced to the game, having fun playing a Life cleric with a pig and the actor feat and then buying their own PHB.
RPGs are not boardgames, and have different requirements and concerns. Even in boardgames, except the ones that earn the title of "classic" or "gateway game" eventually go out of print and disappear. In boardgaming circles, it's referred to as the "cult of the new," as the new hot games replace the hot games from just a few years ago. Admittedly in the US, most people think of the classic games you mentioned, but that is slowly changing as the general population becomes aware of the boardgame renaissance that's been going on for the last decade.
As for RPGs, they have a shelf life that will eventually expire. This was on display at the end of 2E, which is the only edition I've seen that overstayed its lifespan. Most people like 2E, but were ready for 3E when it came out. It updated a lot of clunky mechanics that were baked into the game (which many people had already houseruled). It added new innovative new mechanics that could not simply be added into 2E, such as the Feat system.
Unfortunately, the trend so far has become shorter life cycles for RPGs, which is a bad thing. OD&D lasted only about 3-4 years or so, but it was a rough system built as the concept of RPGs was formed. 1E lasted 12 years, 2E lasted 11 years, 3E lasted 8 years (of which 5years was 3.5E), and 4E lasted only about 4-5 years. BECMI was the longest lived, going about 23 years, over which time it had 4 different iterations (but only 2 major versions, IIRC). This is the trend I want to see bucked, and 5E is doing a hell of job with it. At almost 4 years old, it has no signs of slowing, and I fully expect it to last another 4 without any edition updates (such as a 5.5E). After that I don't know, but hope that it will be the longest lasting edition other than BECMI.
I could see very minor changes made in the future to the core books. Perhaps a feat added or dropped here or an additional version of a class there. Maybe a "Complete" Players handbook that at some point consolidates all of the class and feat options. But no core rule changes. Nothing that would mean that someone with an original PHB wouldn't have the same mechanics for as someone with a PHB printed in 2030.
And they certainly won't call it 5.x on the cover. Sure us hardcore fans will notice and discuss it, but to the public, it will still just be the D&D PHB. They don't even brand 5e on the cover of the core books now.
Unfortunately, as I said that's not the reality of things. At the very least I'd expect a 5.X version by 2030, which as I said, I would hope to be backwards compatible. You are correct they wouldn't call it 5.X, but perhaps PHB revised or something.
Will that happen? 2017 was a stronger year than 2014. While the growth will decline I imagine the customer base to buy new books will be far higher than that of a 6e. Well, at least the risk will be there. Plus it is better to have a stronger brand to support movies which are worth much more than some core books.
I don't think the edition really effects the "brand" concept that much, unless you have a massive shift, like what happened between 3E and 4E that divides the player base. The movie (if it ever happens) won't be tied to an edition, but simply the concept of D&D. Most of the video games they've done have only paid lip service to the rules of any edition of the game. Because of this, I don't think a move to a 6E would significantly hurt the "brand."
Right now 5E is doing great with bringing in new players, which means more core books purchased, and hopefully that continues for a long time. Eventually, however, you will run out of new players buying core rule books, or at least the number will slow enough to become an issue for WotC. Assuming you will always have a huge influx of new players was one of the flawed premises of 4E.