The aging fintech leader still faces plenty of long-term challenges.PayPal's (PYPL 2.30%) stock has rallied 41% this year as the digital payments leader has attempted to put its struggles behind it under a new CEO. Is it still worth investing in anticipation of a long-term recovery? Let's take a fresh look at its business model, its most pressing challenges, and its valuations to decide. What are PayPal's most pressing problems?PayPal owns one of the world's largest digital payment platforms, but a lot of i ...