there’s things to be critical of 5e for. But I don’t think The caster/martial ‘issue’ is one. It has been primarily resolved. Both complaints from 3e and 4e were heard and fixed. The only complaints left are from people that want fundamentally mythic fighters - while slot don’t want that at all.
and while casters and martials aren’t perfectly balanced especially in higher tiers, through tier 2 it’s mostly close enough.
The martial/caster balance issue is really an ability score imbalance issue.