Being a FileMaker developer I feel your pain, to various degrees. Just speculating here, as many in the community have requested a thin-client, Claris has been waiting for Apple to bring iOS apps (FileMaker Go) to macOS. Hopefully Android is on the horizon for FileMaker Go and Windows users will get the same functionality. FileMaker Go is currently free, and though that may change, I’d wager Runtime solutions will continue working until this is sorted out.
If you have people that need to use FileMaker in remote locations, and have an iOS device, you can simply email them a copy of a FileMaker database file and they can open it with FileMaker Go at no cost. If you want to get fancy the FileMaker Go database can exchange data with a database hosted by FileMaker Server using its native data API.
Also not an apples-to-apples replacement for Runtime, with a learning curve and some related costs, but Claris provides an SDK that allows FileMaker files to be converted into iOS apps. I haven’t had the opportunity to try this but there are vendors trying to make this easier, like 24usoftware.com/xgode.