Official figures are hard to come by but estimates suggest that NeXT sold about 50,000 units over its entire lifetime. The company had to stop manufacturing hardware and was very much dying.
You could classify the deal with Apple as a merger in that both companies were in a certain amount of trouble by that point and needed eachother (or some other merger partner) for survival, but Apple had a lot more runway. They did, after all, hand over $429 million in cash for NeXT, plus some quantity of shares.
NeXT technology didn't just overcome technical debt, it became a solid foundation for over two decades worth of cutting-edge software, hardware and product development by Apple. It remains one of Apple's greatest technical assets. But again, the technology alone would not have fundamentally changed Apple's 1996 market position.
Apple's market position was turned around by Steve Jobs, the "freebie" included in the NeXT acquisition.
The software was fantastic, and thankfully Apple acquired NeXT, but let's be honest: if NeXT hadn't been acquired (by somebody) their days were numbered.