It's difficult to say what the future will bring. Other than speculation I'm not sure of any hard info regarding how long amd plans to use am4, through how many cpu releases/models. It could be a platform they stick with for the next 5yrs like am3 or it could be something they change in another year or two to am4+ or am5.
As for intel, they typically change out the socket and board every 2-3 cpu model releases. Currently they use lga1151 and have used it for skylake (6th gen) and kabylake (7th gen). 8th gen coffee lake will likely be on 1151 as well. Cannonlake, not sure it's been decided yet what platform it will run on.
Not sure why you're considering 6600k specifically, you're setting yourself up for an already available upgrade since the 7600k already exists. There aren't as many options as you think either way. Say you get the 6600k, what all future proofing is really there? You could upgrade to a 6700 or 7700 i7, maybe to coffee lake depending on whether it's compatible via bios updates to whatever board you go with now. Most people wouldn't upgrade one generation, they would instead upgrade from an i3 to i5 or i5 to i7. Starting with an i5 you've got the i7. If you hang onto it for 2-3yrs chances are there will be something better out by that time needing a whole new motherboard.
The same is true of ryzen, they have (at the moment) 3 basic cpu's. r5 4c/8t, r5 6c/12t and r7 8c/16t. Each have a couple versions with minimal clock speed differences but that's the gist of it. Just like intel, sure they have a couple i5's a couple i7's, i3's etc. but basically you have a 2c/4t pentium/i3, 4c/4t i5 and 4c/8t i7. With the whole lineup of 1151 compatible cpu's there really aren't that many choices with intel, there might be 6 different i5's but they're nearly identical. The biggest difference, k series overclock and non k don't.
The takeaway is neither seems to have more choices than the other in reality unless arguing over there being more cpu's because intel has 4 of them 100mhz apart. How much upgrade potential depends on how high up the ladder you start out. Someone who begins with a pentium could theoretically upgrade to an i3, i5 then i7. Starting with an i5 means the only one left is the i7. With amd, if you start out with a low end r5 you could upgrade to a higher end r5 or choose from a couple of r7's (which are nearly the same when you consider the 1700, 1700x and 1800x).
My personal preference is to make a solid choice to begin with given the budget you have to work with and consider sticking with it for awhile. Starting low and making tiny incremental upgrades hitting every cpu up the chain doesn't make much sense to me. Minimal performance gains and a loss of money, not to mention time in a perpetual upgrade frenzy of chasing your own tail.
If you start with 7th gen, a pentium g4560 for around $90 then move to a k series i3 for $180 then move to an i5 for $230 then move to an i7 for $350 you'll have spent over $800 in cpu's. Even if you sell off the old ones no one is going to buy them for what you paid for them, so consider every cpu swap selling the old one for at least a $20-30 loss. In the end you'll have spent a good $120+ in lost investment and time tearing out and reinstalling cpu's for minimal margins of performance. Just illustrating what a real world 'upgrade rich potential' really looks like and that it's unrealistic.
Either setup uses ddr4 ram so upgrades moving forward should be able to reuse the memory no matter which platform amd/intel you choose. Upgrades shouldn't be any harder for one over the other.
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