More startups fail from not enough money coming in than from having tech fail. Many mid-life startups fail because their technical debt causes them to be unable to quickly grow and respond to changing conditions.
How to get the speed of development with short-term thinking but avoid long-term pain?
* Keep it contained. The big-picture diagram should be pretty good, but the details of implementation can be shitty as long as the same interface would accomodate the better version. This sets you up for success later when you need to replace the bad implementation with a better one.
* keep your core clean. The more features and other code paths rely on a chunk of code, the more 'core' it is. "Core" code should be kept clean because of the risk that bad core code poses to the business.
* comment your regrets and thoughts about how you'd fix it in the code so when you burn out and some new hacker is trying to fix your crap they will at least be able to have an idea of what you were intending to do.