That doesn't mean it's a done deal. House Republicans will likely obstruct the admission of a new solidly-blue state. Puerto Rico's population is 3,700,000, slightly bigger than my own state of Oregon, so it would have seven or eight electoral votes and five or six House seats, along with two Senators. The effect on the national party balance of power would be small but real, and that's all that the Republican establishment seems to care about these days.
I'm personally somewhat hesitant about the idea. We haven't admitted a new state since 1959 -- the age of expansion is long past and the country's borders are now, in most people's minds, pretty much fixed. And we've never admitted a state in which the majority native language was not English. But this is not so out-of-left-field as, say, annexing Cuba or British Columbia. Puerto Rico has been under American rule since 1898 and its people have been US citizens since 1917. Its territorial status is something of an anomaly -- not foreign soil but not quite fully a normal part of the US either. Now that the people have voted to regularize their status, the issue deserves sober consideration.