First off, you're not "fudding". You're just curious; which is actually a good thing. Rather than being the "bitcoin is going to moon even though I don't know crap about it!" kind of person.
But what is to stop another 'gold/value storage' coin with better technology than bitcoin taking over.?
Nothing. Nothing is stopping a better coin to take over. Though while it's possible, it's definitely unlikely in my opinion; especially in the short-mid term. Bitcoin is simply so far ahead.
As - I could create a value storage coin with only 10 million maximum circulation, thus trumping bitcoin as a coin of value with inherent rarity.
Not because the coin you made is more "limited", it doesn't really make it better. One of the biggest things that makes bitcoin heavily better than most if not all coins, is it's network effect. Bitcoin is one of the most developed cryptocurrencies, and the most adopted. You could also technically create your own version of the dollar, the "goat" dollar; probably with lower inflation rates. But, who's going to use or accept it? No one. Same thing with bitcoin.
To top that It could have modern improvements that some of the other altcoins have these days - instant transactions and better scaling (no idea what that is btw) e.t.c.
To be fair, those coins with "free" and "instant" transactions has a lot of downsides to them. Remember, speed and fees isn't the thing we should prioritize when looking at a new version of money. Decentralization and security should definitely be the first priority, not speed and fees.