>> there is not enough budget to actually rewrote whole system,
As pointed out elsewhere, this is definitely solving a real problem, the longevity of the app is the proof.
Instead of rewriting, can you replace it with newer idioms? A MVP/PoC for a newer way of solving a problem (AR may be here) that the software solves with some tangible gains, the latter is more important, can lead to approval of a mini budget for that MVP and who knows what that can lead to.