a BETTER coin, one with improved technology can replace bitcoin .... the improvement must be very significant and bitcoin needs to stagnate and fail to improve over a very long time....
If a coin can be replaced over night by another coin with better technology ...nobody is going to want to hold any significant value in any coins....People will stop buying them. If there's not faith in a currency staying around and holding value it won't hold any value at all.
You really need to distinguish a variety of cases and phases of development in order to apply that reasoning suitably.
Firstly, we should distinguish application tokens and financial engineering vehicles from cash money. The end-game for cash money has a well-defined fundamental valuation, with low model risk (Fisher quantity theory), whereas the model risk for app tokens and ponzis is vast, and the best case upside potential is much curtailed relative to cash money. All crypto is now primarily a speculative vehicle, but BTC at least has a fundamental economy of some size, and that won't change suddenly.
Secondly, we should distinguish the various levels of maturity of cash money. Gold is mature. Debt-based national currency under centralized management is experimental, but ubiquitous. Bitcoin is unknown to 99.95% of the potential user base, and lacks essential features, and development is stagnating. Monero is unknown to 99.9999% of the potential user base, and lacks essential features, but development velocity is quite respectable. Clearly the growth potential of XMR is much greater than any other instrument in operation today, if you consider the prospect of becoming the primary international medium of financial exchange to be within the realm of possibility.
The best example of crypto achieving bitcoin mcap is bitcoin, and that took 5 years. It takes time for money to flow, and the pipes are narrow enough so that back-pressure prevents too rapid a rise in mcap for XMR. It would take perhaps $200mm influx to bring XMR to parity with BTC. Poloniex can handle perhaps 20000BTC/diem, max, so it would take at least two months for parity to be achieved,
if everyone decided today that BTC was utterly uninteresting, and only XMR mattered. So, if XMR will be successful similarly to BTC, with no less speed, it will take between 2 months and 3 years to get there. Meanwhile, there is no particular reason for BTC to decline while XMR ascends. Quite the contrary, in fact, as buying XMR requires first buying BTC. By the time XMR reaches USD 420, BTC will almost certainly pass $1k - unless a reliable and accessible fiat exchange or three opens meanwhile.
No, if everyone decided today that BTC is utterly uninteresting the price would very quickly reach 0. And there would be nobody buying BTC to exchange for XMR. No sane XMR holder would sell for BTC. The BTC/XMR pair would be rendered useless on the spot and at best fractions of XMR would be sold for BTC in the final hours of bitcoin's death.
If we extend the time period it takes for most people to lose interest in BTC to something longer like 2 months you'll still have early speculators dumping BTC to preserve their wealth. Some will dump for USD and some will dump for XMR. But a few enlightened speculators would likely be enough to cause major panic and havoc to the cryptocurrency space. The smartest speculators will recognize the fallacy that will haunt the space, namely:
A coin can easily be replaced with a little improvement in technology with no ill effects on the economy. This is a fallacy because if that is the case there will be a race to the bottom in every following coin in the dawn of a new one. Competition will drive the time it takes to replace a coin down from perhaps 3 years today to 1 year, 6 months, 3 months, 1 month, 2 weeks, 1 week, 1 day... At some point coins are replaced so often that they can no longer hold any value. What is the properties of money? One is store of value. Which will be no more. Without store of value we can't have a medium of exchange. Without that, there is no currency.