Thanks to the Lightning Network, it's now possible for Bitcoin to provide instant, cheap transfers to everyone. Adoption for the off-chain scaling solution is low, in part because of the LN's complexity. Now, imagine an scenario where the Lightning Network becomes heavily adopted by people in the mainstream world. It could threaten the existence of traditional payment processors as we speak. While I don't know how much TPS the Lightning Network is able to process, I'm pretty sure it's near close as VISA.
Do you think the Lightning Network will be able to challenge VISA someday? If not why? What if traditional payment processors like PayPal, VISA, and Mastercard decide to run a LN node in the future? Will this benefit Bitcoin in the long run or not? Your input will be greatly appreciated.

Nope.
What is much more plausible is VISA to adopt crypto/stablecoins (which they are testing right now) and then to use your crypto on every VISA logo store/shop.
VISA is very pro-crypto and friendly.