It's a question of anticipating supply (scarcity) and future demand for a particular model. You can more or less attempt to gauge the supply side, particularly for models out of production, but forecasting demand in 40 years is crystal balling. These days there are so many niche brands with very low output that I'd ignore them because, chances are, niche today, niche tomorrow, and watches from small high-end manufactures aren't "readily available" anyway.
At the risk of being completely wrong, I'll jump off the fence and give a name. Tudor's Black Bay family is accessible and popular. The smaller size of the BB58 appears to be more popular than the original BB ETA size and the larger still in-house movements. The BB58s and in-house BBs are still in production and will probably remain so for a long while due to popularity, so they won't be scarce. The BB ETAs are no longer manufactured and one model in particular was only produced for a few months - the black bezel (BBB). Even though the BB58 size is more popular today, size is not necessarily a major factor for collectors - many sought-after vintage watches today are smaller than contemporary watches. The BBB ETA may not be so readily available any more but they pop up. So I'm punting future collectors will look for them because they are the holders of the original genes of the popular and growing BB family, they have scarcity and they're a great looking watch with a movement that doesn't need to go back to Tudor for service. You can buy me a beer in 30 years if I'm right.
Here's mine:
View attachment 854372
Click to expand...