I know this may seem out of nowhere

Well Godzilla's first and only movie crossover with a creature outside toho, was probably his most successful movie. I don't know if Godzilla vs Mothra 92, or G14 ended up overtaking it in ticket sales, but King Kong vs Godzilla held the title of most ticket sales for a long time.

That kicked Godzilla back off after Godzilla Raids Again did less than steller and could be attributed to the long list of movies he has. If he never crossed Kong it's debatable and pure speculation where the series would be now, but I find it most likely that the movie list that the series currently holds might be lower.

What about if he never crossed Mothra or any other Toho creature?

^Ultimately, the idea of crossing Godzilla with other Toho monsters rolled from the success of King Kong vs. Godzilla. I don't think we'd really have a Godzilla series today if Godzilla's crossovers didn't happen.

^Not to mention that the other monsters that crossed over into the Godzilla franchise (Mothra, Rodan, etc.) worked really well in the Godzilla universe.

King Kong vs Godzilla will likely always be the king in ticket sales for a Godzilla movie. Even a remake of that film wouldn't have that sort of marquee presence today and the reason is sinply that with television, the internet, blu-ray, etc. people aren't going out to the movies as much. If you look at overall ticket sales, there's only one film from the 2000's that breaks the top 20 (Avatar) and the most recent film to get into the top ten is Titanic and that film was released in 1997.