Over 50% of most companies' traffic is now mobile. Creating another 3 codebases (iOS, Android, and mobile web) for your current application is an insane proposition that becomes more problematic as time passes.
Rewrite one codebase, or create 3 new ones? That's not a hard decision to make, especially when you consider the future impact of being able to add new features in one place.
But even if you decide to migrate your codebase to the Modern Web for all these reasons, organizational and team challenges remain.
And even if you get great training, and your team learns all the new practices quickly, daunting organizational challenges remain.
In short, the opportunity costs of not serving your current market for too long is far higher than the cost of building the new software, and even if you had the budget to hire your missing skills, market realities mean you won't be able to hire those people fast enough.