I know several extremely successful, productive people in different fields who work on the "work as long as you like when you feel like it, rest when you don't" principle.
80 hours still leaves over half his time for eating, sleeping, the small amounts of physical activity which are realistic, and small amounts of human interaction. It simply does not leave time for leisure or to unwind, but as I said previously if your work is what you would do with your leisure time then that's not an issue, as there is nothing to unwind from.
Now I have only encountered this situation for a brief 3 month project which I wish had never been finished. But this type of dream job type situation does exist, and I don not believe is unhealthy, unless you have some sort of family to maintain also (it doesn't really leave enough time for a healthy family life).
Take the analogy of an athlete. Any athlete knows that recovery is an important part of training. Too much work and they over train. This can be applied to any area of life but is usually called burn out in business. Burn out happens regardless of how much one enjoys the work.
And with the example of the athlete a portion of their job is to have down time, and not be out doing strenuous physical activities the night before the big game.
When I encountered the situation where my leisure became my job, I simply didn't have anything I wanted to do to fill the free time anymore. Sure I could of done push ups or gone on dates or something else which I personally hate doing, but making myself miserable in my free time isn't my idea of healthy.
I don't qualify continuing to work as leisure time just because you happen to like your work. Work is work, regardless of whether or not you like it.
Also I don't believe that simply increasing the amount of hours you work on a startup will directly impact its success. Cutting the scope is almost always a better way to improve a startup's chances.
What I'm suggesting is that working 80 hours per week is by its very nature harmful to you, your coworkers, and the company, regardless of whether it sout of interested for the things you're doing. It leads to scope creep, burnout, and other unhealthy outcomes. Everything should be done in moderation, including (especially?) work. I can foresee a situation where you or somebody on your team will ignore the obvious signs of burnout and power through until its too late.
If you feel like you need to work that much I would suggest you re-evaluate your scope or find more coworkers. Look to other companies who have built fantastic products without sacrificing a good work/life balance (37 Signals comes to mind).
We, of course, hope that we will not have to work 80 hours every week down forever - but at an early stage we can't help ourselves. The extreme motivation, energy and drive in the team keeps us running.
This isn't sustainable. It's likely not healthy, either. It doesn't matter how fun you're having while you work 80 hours per week, it's not good for you. Most successful businesses grind away for years before they finally become profitable. The rocket-ship success stories like Whatsapp and Instagram are the exception to that rule. The whole mentality that you need to work yourself to the bone in the name of "passion" to build a successful business is poisonous. Even if you happen to be one of these super humans that can sustain that amount of work more than a year it's likely that your co-workers are not. It's also likely that they will not complain about it because they don't want to seem like the weak link. All the while, though, it will do damage to them and to the company in the long term.
Maybe working 80 hours a week is a sign that you're over-engineering. Simplify, focus, and work less. The product will likely become more focused, less bloated, and have a much better chance of surviving over the long haul.