I can see why some would want to be independent. There are downsides to being in a fixed currency union with the US. Inflation that's driven by a booming economy on the mainland can drive up prices in PR, even if there isn't a corresponding economic boom there to justify it. Puerto Rico currently has to endure the Jones Act, which substantially drives up the price of goods shipped there from the US mainland. They wouldn't be subject to it if they were an independent country.

You mention a military, but as an independent country, they wouldn't actually have to field a big military. Look at the other states in the neighborhood. The Dominican Republic is a good model. They currently spend about 0.7% of their GDP on the military, the US spends about 3.5%. And that's what they spend sharing a giant land border with Haiti, a completely failed state. Most of their military is on that land border trying to keep people from coming across. As a whole, military spending in Central America and the Caribbean is quite low. The whole area is in the US's backyard, and the US is never going to accept some other nation going on a warpath through the Caribbean. If tomorrow some later-day Napoleon takes over the Dominican Republic, and they decide to invade island after island in some grand imperial war, the US is not going to sit back and just let that happen. For the independent nations of the Caribbean and Central America, the US's generations-long policy, overt or covert, is, "don't worry too much about overt military threats from your neighbors. If anyone actually threatens your borders, we'll stop them. Stay in our trading sphere and don't ally with adversarial powers to the US, and your security is assured."

Realistically, an independent Puerto Rico would have zero external military threats to worry about. What limited military it would need would mostly be spent protecting its territorial waters from illegal exploitation, or in preventing migrants from coming in from nations undergoing severe political discord. But if anyone ever tried to invade them, the US would certainly step in.

In fact, their only serious threat from invasion would come from the US itself. If an independent Puerto Rico decided for some reason to seriously ally itself with China, and let the Chinese Navy set up a huge base on the island, or something similar, they could end up as a second Cuba.

But as long as they don't do that, they would face few security threats. Realistically, like other island states in the area, an independent PR would need very little military spending. Hell, the US would likely pay for the entire PR military through generous security assistance grants provided in exchange for letting the US keep military bases on the island.

If the citizens of PR want to go full Cuba - seize all the tourist and other assets held by mainland investors, become friendly with Russia/China, go fully overt socialist or Communist? In that case, independence would likely turn out very badly for the future of the island. But if they want to become independent, but just take on a roll very similar to the other independent island states in the area? - Remain friendly to the US, keep trading with the US, largely rely on the US for protection from overt military threats, etc? They could actually do quite well by independence.

Of course, there are advantages to statehood as well. Having your citizens fully eligible for all forms of federal assistance, when your population's average wealth and income is well below national averages? That has some advantages. It would allow PR to give welfare benefits to their poorest people at a level of generosity that they could never afford to do as an independent state. Plus having representatives and Senators can't hurt. Smaller states often are able to divert federal spending to within their borders in exchange for a vote in the Senate.

So really, I can see valid arguments on both sides. But personally, I think it's time we settle the matter. I'm in favor of Congress passing a law which directly forces the issue. Pass an act that grants either full statehood or full independence to Puerto Rico - and make it entirely contingent on a final binding referendum. I think territories like PR are an anachronism in this day and age, and I think the US needs to get out of the business of holding heavily populated territories. I think our territories, at least those with any more than a trivial number of people on them - should either be granted independence, be fully integrated as proper states, or be folded into existing states. I don't mind some tiny rock with 5 people on it remaining a territory indefinitely, but we shouldn't have territories where thousands of people live on them without full representation. I'm in favor of passing laws that force the issue on all of our present territories. Personally I would be fine giving the US Virgin Islands the choice - join PR in statehood or become independent. For the ones in the Pacific, the choice could be - join Hawaii or become independent. Or maybe we could just fold all the remaining non-Hawaii states into a single new state called "The State of Outlying Pacific Islands". I'm sure the first act of that state's legislature would be to come up with a better name for the place.