It is difficult to know, the code of bitcoin is open source so eventually altcoins would have been created, but if for some reason that did not happen then all the money currently invested in the whole market will be in bitcoin, it is also likely that the developers will be pressured to add more functions to bitcoin than what we have now but it is doubtful if the developers could make the best possible job when facing such pressure.
Despite the despise that some members of the forum have for altcoins the truth is that we need them, bitcoin cannot do everything and it is OK to have other coins specializing in certain aspects like privacy to allow bitcoin to perform its main function which is to be a currency.
I see it in the aspect of connected market volatility. Without altcoin, bitcoin will not have a large growth in periodic estimates, without altcoin also it will not make economic instruments on the exchange have a solid rhythm. bitcoin will have a fixed level of stagnation and will lose value in the long run if it stands alone without supporters.
There are lots of altcoins who really have deserving high place like bitcoin just like ethereum, as we all know, ethereum has a solid platform that is why it is become so famous until now, having a smart contract in which you can do your own transaction in a private without hiring any lawyers, it is fast and secure transaction. There are other altcoins that has solid platform and some of them are blockchain-based, meaning they are using technology to innovate their system to make the investors really satisfied and not only them but also to satisfy the user of that particular platform or technology.