Why can't any number of alt chains and Bitcoin all get along? There are numerous fiat currencies in the world, from the Swiss Franc to the Singapore dollar. The way we handle them is by exchange rates (duh). Bitcoin, Litecoin and possibly hundreds or thousands of others can all live in the same world and be traded in the future. Why couldn't there be as many crypto-currencies as their are stocks traded on global exchanges? There can be, if you ask me. And as with stocks, people will have to examine the merits of each one individually and decide which ones are worth "investing" in. In fact, I think it's better this way. Newer and better types of currency can replace the old and an ultra-modern economy can flourish without the need for centralized fiat currencies.
That being said, I'm actually speculating on exchange rates between alternate cryptos, Bitcoin and fiat currencies... I'm a FOREX trader, so I can't help myself. If I told you what I was doing you might think it a "high-stakes gamble", but I'm not risking what I can't afford to lose and the payoff could be huge. I do the same sort of thing with risky S&P 500 and VIX futures/options, so why not crypto currencies? If you know what you're doing, speculation pays...
Because we have one planet, so far.
The multitude of currencies is because of the multitude of countries that force them to issue their own national currency, but that has been changing with the evolution of free trade areas and further evolution in the creation of monetary unions, which further reduced the number of currencies in circulation: namely the Euro, which ended up replacing 17 currencies in Europe. Central and West Africa have Economic and Monetary Union, which comprises 14 countries in Africa using CFA francs. The North American Union, proposing a new currency the Amero for Canada, US and Mexico (not implemented yet), and soon the Sucre currency for the Alba, replacing 8 currencies.
It is an evolutionary trend, the better the interrelationships, more common markets, more economic unions and more monetary unions, implies less national currencies we'll have in the future. In short, this world is becoming globalized.
Bitcoin doesn't belong to any country nor region, it is global by default.
It is unnecessary to have many digital currencies when they don't have a clear differentiating purpose.
Let me remind you the purpose of money: a medium of exchange, a unit of account, a store of value.
If you are cheering the creation of alternative cryptocurrencies solely for speculation, then such currency is sterile from the get go.
That's when we'll have our first tulipmania and we will ALL suffer, even the BTC will take a hit damaging the reputation that has been building painstakingly.