That's true, but to a point.While this can be true if you actively trade u can beat the rares over time but it is a lot of work compared to buy and hold strategy u r proposingGold loses value overtime, it's actually kinda bad to have a lot in gold. Always go with rare/high demand items if you have a choice. Then you can gain value from doing nothing, instead of having gold that you won't spend laying around.
Merching can only go so far(limit of stuff for sale, dry times, etc). Most merchable items don't have that much demand, or if they do they usually don't get much profit.
Rares(provided you buy the right ones), will always have demand, but will slowly gain value. This of course jumps up temporarily during a double plat event, but most times it wouldn't be worth it to sell them.
Merching is better with a lower amount of gold, but when you are dealing with hundreds of millions in rare items then that will surely gain gold faster(without having to do anything too). I'm not saying you can't do both though, I do both personally. Just save about 1-3m for merching and spend the rest on rares to hold.