For a quick answer: ALWAYS, never quit doing online marketing.
It's sometimes hard to justify a cost when you hear the same thing time and time again; "SEO is not quick, it takes time." Trust us, it is worth every penny. You should always have an internet marketing budget and SEO should be at the top of that list. Just because you reach to top of page 1 in Google, doesn't mean you can let up.
Getting to the coveted 1st page of Google results or even becoming the king of the hill at the #1 spot is a marathon; not a sprint. And once you finish that race, don't retire. Meaning even at the #1 spot, SEO is still important to focus on. Your competitors will see you at the top and continue to work until they over turn your position. It's a dog eat dog world in the internet marketing world.
Google is a business that wants what's best for their clients. They don't care specifically about your rankings, they care about returning relevant results. To achieve this, they are constantly tweaking and changing their algorithms based on searches, platforms, devices and thousands of other items. This means the SEO company that you choose, HAS to offer a multitude of services including changes to your actual site. If anyone promises you rankings, be leery of those promises. SEO should and will always be a service that is ongoing and constantly evolving.