I suspect it's less Spotify's unwillingness but instead structural difficulties on the Apple Watch platform. Maybe Apple isn't interested in allowing a competitor...
As a point of evidence, Marco Arment's podcast app had to remove Apple Watch support after the approach he had used was eliminated[1-2].
That makes complete sense based on my personal use. I have a Music subscription purely because I can use it with my series 2 watch offline. I use Spotify the rest of the day. If Spotify had a watch app where I could download songs offline, I'd drop my Music subscription immediately.
I am curious... what would be the benefit of offline music in a watch? AFAIK you can play downloaded songs in the smartphone with Spotify while offline
The Series 3 with LTE is even better for runners than the Series 2 offers.
Many stores and vending machines accept Apple Pay if emergency food/water/medicine is required on the run. You can also make phone calls or hail an uber/lyft in case you twist your ankle or another a different minor injury while running.
The combination of offline music on the Apple Watch plus AirPods is the first thing that's improved on the experience of the clip-on iPod Shuffle and headphones. Despite some UI lag issues it's basically running nirvana.
iOS, Apple Watch, HomePod, Apple TV all have limitations that prevent competitors to imitate or improve on the Apple Music experience. The lack of Spotify support is the reason I didn't buy an Homepod when I was in the market for a speaker system for my office though, so I'm not sure if it's working for them.
As a point of evidence, Marco Arment's podcast app had to remove Apple Watch support after the approach he had used was eliminated[1-2].
1. https://marco.org/2017/09/24/what-watch-podcast-apps-need
2. https://marco.org/2017/08/10/removed-send-to-watch