I don't think Monero will ever achieve mass adoption. Those in power aren't going to relinquish their control in favor of a decentralized model without a fight. Monero exists, and will continue to exist, in a pseudo-economy. Other currencies gain adoption because they are traceable; in other words, they are accepted by those in power because they can control them. Monero, on the other hand, is the true embodiment of Bitcoin, preserving Satoshi's cypherpunk spirit.
Discussion
It's a Trojan horse. Every other coin can be swapped for Monero atomically.
Monero will either be the backend or backend and frontend of the cryptoeconomy that will steadily eat the fiat economy.
The plutocrats tried hard to fight Monero and they lost. The rest is history in the making. Not everybody can see it yet. But it will unfold before everyone's eyes.