You may be able to carry on with your SEO without on-page optimisation, but there’s a limit to how far you can go. Trying to do this is like driving a car without fuel, or building a house without a proper foundation.  In the same way, all your effort to build a successful SEO campaign will be frustrated without having the proper foundation which is on-page optimization. It may cost you to do your on-page optimisation, but you’ll benefit from it in the long run. Not only that, but if you skip it now, you’ll still have to optimize your on-page for SEO in the future.