1. New business are opening all the time. Every. Single. Day.
2. Most businesses go looking for solutions only when something is very painful. Convincing them of the need is something a lot of existing SaSS businesses don't do as there's so much low hanging fruit around still. If you can send a salesperson in, you win by default. You'll be amazed at how much you can charge for something.
3. Lots of businesses (the majority) are 5-10 years behind in the computerization of their business compared to any business you find mentioned here or any business you'll ever speak to at networking events for startups. That's pre-SaSS.
4. There are a lot of businesses where the internal IT dept wrote all the 'apps', they usually suck and are broken. More and more businesses are giving up on this model and buying the app off-the-shelf. A little customisability (logo, maybe colours, even name) goes a long way.
5. A lot of businesses still rely on some crappy Sharepoint or even Lotus or [insert unwieldy enterprise system here]. The internal people hate dealing with them and will switch to your system if you find someone with enough clout to sidestep 'the rules'. A 'department trial' is also a handy way in, some companies have very autonomous departments.
6. There's no such thing as an established SaSS businesses outside of the echo chamber of HN. Many businesses will not have heard of Salesforce, let alone basecamp. There's no such thing as an established time tracking tool for example.
7. Some 'great' apps are far too generic. New to do apps still come out every month
8. Marketing and designing your app for a particular market sector can give you a huge advantage over others. Just knowing the right words, phrases and acronyms to use gives you huge credibility with that market. Compare 'Project Management' to 'Project Management for Architects with full support for Drawing Issues and Revision Tracking'.
Before you embark on doing anything though, you have to accept that any business app you write is going to be incredibly complicated and you're going to have to fight to keep it simple as everyone wants to use it oh so slightly differently.