OK, I'm old, but I remember when there was no such thing as a cycling computer, or Garmin, or Strava. World-class cyclists trained with nothing more than a watch and maybe a stopwatch. Technology has provided some great tools, but we need to remember they are adjuncts, not replacements for good training strategies and learning to listen to our bodies. Can you train without the latest and greatest GPS, realtime mapping, heart monitoring, cadence counting, watts calculating, technological wonder? Heck yeah. All the technology does is put numbers to things you should already know. They can quantify but they can not qualify your efforts.
I like having at least a basic computer for the majority of my riding but not because I couldn't train without it.