I've been married twice and have had more than a few intense, passionate, and long term relationships. Yes it is possible that your wife will meet a man who fucks better than you, who is more suited to her, who could even love her more than you. The thing is she could meet him at any time, anywhere, not just in swinging. It happens.
None of my relationships broke up because of swinging and I've swung with most of them. Relationships and marriages generally break up because there are everyday living issues that strain it to the point of breaking. In other words, you weren't really made for each other and it is better to separate and find a more suitable mate.
There is a belief among some swingers that love is proof against your mate falling for another. I don't agree - swinging has little to do with it. Those who don't swing fall for others in life. Those who do swing fall for others in life. Swinging isn't the factor. Except that swinging often helps to strengthen communication, highlight areas of possible improvement that allows you work together toward a finer marriage or relationship. Swinging also can deal with and lessen the "I want to fuck others" tensions that affects almost every marriage.