I've had the experience of buying a bike that didn't fit and then spending a lot of time trying to make it fit. It was my first real road bike (ignoring my old Univega that I still have). I had tried a few bikes at the time and one bike I tried (I believe a Cannondale R600?) fit better, but was more expensive than the one I got (Lemond Tourmalet). I never did get the Lemond to fit.

A couple of years later, I decided to buy a new bike and spend a good amount of time just riding a lot of bikes over and over again, with different materials and geometries. The last day I did this, I had three bikes that made it to the finale, an Orbea, a Felt, and a Cannondale. It then came down to Felt and Cannondale, and I really tried to convince myself that I could make the Felt (all Carbon) fit as well as the Cannondale, which just disappeared under me. In the end, I picked the Cannondale (R1000) and know I made the right decision. It may have been possible to have a fitter fit the Felt to me, but I did not want to take the chance. I've comfortably ridden centuries on the Cannondale with only minor tweaks.

So in my case, it was a matter of taking time to the point where I knew the bike would fit. I may still get a fitting some day, but I'm not sure it would result in anything more than a small incremental improvement.