Drilling a hole through concrete is not a terribly difficult thing to do. Even one 14" deep. Long masonry bits exist at hardware stores for exactly such purposes (well, intended for thick walls rather than floors, but close enough).

That remains the best technical solution.

Next up, drill 3/16" holes through an upper story window frame, and a lower story window frame, and run the cabling through those. Also a very good technical solution.

The problem with most consumer grade Wifi gear is that a lot of it tends to run HOT. Which can be a problem for setups like the current outdoor repeater. A purpose-built pole-mount style of repeater might deal better with the elements, but those do tend to cost as much as 3-5 of the current WRT54GL unit out there.

Another idea I had, was to locate the upstairs wifi next to a window with a really good view of the lower balcony/terrace. And place a suitable metal object down on that balcony to reflect signals between that wifi unit and the suite downstairs. Might work, might not. Easy to mock up and try with some foil wrap or a metal patio table.

As for the existing routers, like most electronics, all that can/will fail are the electrolytic capacitors. Most likely in the power supply dongles before anything else. So just replacing those relatively cheap items can often extend the lifetime of the gear by another few years.

Cheers!