tl;dr
Why try and make Bitcoin do everything instead of having an ecosystem of specialized currencies?
One Coin to rule them all, One Coin to find them,
One Coin to bring them all and in the darkness bind them
And that, my friend, is why bitcoin was created.
Joking aside.
Isn't the situation you are talking about is happening right now? We have over 6000 altcoins, at least judging by ANN topics from the Altcoin section.
700 or so is listed on coinmarketcap and 100 coins are over $1 million marketcap. Still, most of them offer nothing interesting and are waste of time and effort.
Most of them lack the hashing power that would be needed to withstand a sustained attack by a state level adversary wanting to re-write transaction history.
Mining power is extremely important. One of the drawbacks to promoting side-chains for the bulk of transactions is it removes the financial incentive to professionally mine, reducing the security provided by the hash power.