Good analogy between MIDI and Soundfonts. And VST for that matter. Despite their limitations they continue to be widely-used because they are public standards. The Kontakt environment may be light years beyond soundfonts, but it's proprietary. There will never be a third-party Kontakt player.
Plogue sforzando, though basic, lets anybody experience sfz playback. It's not multi-timbral, it doesn't come with a big collection of instruments, the UI is simple and updates have been sparse (after 5 years it's still version 1). But it's FREE and 64-bit.
And of course, there's CW's own Rapture.
It's actually possible to dive deep into the world of sampled instruments and never set foot in the Kontakt universe. There have been a number of quality commercial libraries adapted to sforzando. Here's an example, a multi-miked Kawai concert grand piano (pianos are especially well-suited to the format):
You probably already have more sfz instruments than you realize. Rapture is mostly sfz-based. Many Kontakt libraries that you'd assume require full Kontakt, e.g. the cheap Iron Pack instruments from SoundIron, are available/bundled as sfz. I'm always raving about Indiginus' Solid State Symphony but forget to mention that its "lite" sibling SSS_Q ships with an sfz version in addition to the Kontakt instrument.