Suppose Monero (or a different alt) were to gain enough traction for it to appear inevitable that it would eventually topple bitcoin. How feasible would it be to fork bitcoin to adopt some or all features of Monero? iow, a merger of the first mover advantage of bitcoin with the technical advantages of Monero. I suppose this question has two parts: 1) What would be the technical feasibility of doing this? and 2) Would the community go along?
The chances of such a merger are zero. From a technical perspective the coins are very different, and the economic interests strongly align against this. More importantly such a merger would violate the most basic economic fundamentals of both coins. This means that such a merger would break all trust in the coins and yes could make them both worthless. Both communities would reject such a merger and for very good reasons.
What is more likely to happen here is that Bitcoin would have to fork to deal with the 1 MB Limit. Both coins would live side by side, compete and hopefully learn from each other. First mover is not everything. A very good example is the credit card industry. The first movers were Diner's Club and then American Express. Then came the later entrants including Visa and MasterCard. The first movers are still around but with vastly reduced market share.
The challenge for the Bitcoin is to deal with the 1 MB blocksize limit before the above happens and not become the American Express or even Diner's Club of crypto-currency.
Edit: Bitcoin might even fork in the middle of a boom, like it did the last time. After all it is way easier to get consensus when everyone is getting wealthier.