No one should be under any illusions that the air travel market isn't under some pressure right now. It is. Still, the long-term case for investing in a higher-quality airline like Delta Air Lines (DAL -4.88%) remains undiminished, and some of the forces that created the weakness are now strengthening the case for buying the stock. Here's why buying Delta stock can set you up for many years of profitable income.Near-term weakness, long-term strengthUnfortunately, the escalation in trade conflicts and tariff ...