Paypal cannot be overcome by bitcoins because of the simple reason that 90% of the online e-commerce sites accept paypal as a payment method and hence that plays a vital role. Though a person who knows btc and paypal both would always prefer dealing via bitcoins.
If 90% of the people in the world pay for things with PayPal then PayPal has a very large chance in being the top payment processor for online transactions. People that use Bitcoin will see how well PayPal is performing and will try copying it though I am sure that Bitcoin will fail in doing so.
If PayPal integrates Bitcoin into their PayPal system then Bitcoin will be much more popular than PayPal.
Yes you are right that many people are using PayPal but it is also true that now many people are adopting Bitcoin and they like to do their deals in Bitcoin and pay for their transactions in Bitcoin. So it mean that PayPal is going down and people prefer to Bitcoin and so it shows that Bitcoin will soon overcome PayPal.