> Cryogenics and wormholes are the two best future options

I'd argue that there's a third option that's far better understood and has already been mentioned a few times. Biological immortality. If we can find solutions to problems like aging and disease, problems we're currently researching and resolving, it raises the real possibility that human lifespans could be extended on a near-perpetual basis. Yes, there are a lot of hurdles to overcome, but there's no reason why human lifespans couldn't reach 100,000+ years at some point in the distant future. Fix the problems that kill us, and figure out how to replace the parts that wear out.

Spending 2,500 years transiting to another star system isn't such a big deal if you've got a projected lifespan of 80,000 years.

Land drying tends to be a major cause as well. Many low-lying islands are made up of boggy sedimentary soils slowly deposited over thousands of years. Boggy soils contain a lot of water. When humans drain that soil for farming, the removal of the water allows the remaining soil to compact and settle. If the land is low enough, it can fall below the surrounding water level and the island will vanish.

The Delta in California has this issue today. When Europeans first mapped it in the 1700's, they found around 60 islands with fertile soil. The islands were surrounded by levees and farmed starting in the mid-1800's. Today, all of the islands are at least 10 feet below sea level, with some of them approaching 30 feet below sea level. Only the large modern levees and constant pumping keep them dry. Some have failed over the years, converting those islands into open water. This will likely happen to all of them over the next 100 years, allowing the San Francisco Bay to extend itself all the way into the Central Valley.