Your particular example made me smile. I'm currently leading a team whose primary product is the front end service for the core SaaS business that does billion

Your particular example made me smile. I'm currently leading a team whose primary product is the front end service for the core SaaS business that does billions of monthly impressions (hundreds of millions of uniques). This product is over 10 years old, written in Spring, and has around 400,000 lines of Java. A couple years ago, we underwent a massive overhaul to the system and refactored the application to use our realtime steaming platform instead of using MySQL+Solr. It took some time to get it done, mainly because at the time there were about 8 years of features baked into a monolithic web service. Ultimately, we were successful in the endeavor. It took about 2.5 years and our response times are dramatically faster (and less expensive in terms of infrastructure cost). So yeah, you can do it. It's expensive and takes time if you wait too long. And you need full support from the business. And some companies don't survive it. YMMV.