The issue is not WOW Air the issue is dumping capacity in already over saturated markets where they have to basically give seats away to fill planes. There are many untapped markets where both these airlines could coexist comfortably but when you both compete for a market like CLE and DFW that have no track record that they are viable you are just asking to accumulate losses and one airline using a widebody and allowing it to sit on the ground for almost 24 hours making no money. WOW should have stuck to markets where they could go back and forth to KEF without aircraft sitting on the ground not making money. CLT, RDU, and the like where FI wasn't flying WOW Air is a great airline for college towns where young people could go out and see Europe.
No, the real issue is that the transatlantic leisure market is highly seasonal. The somewhat less seasonal high end of the market is captured by the legacy carriers/alliances with their business class products and breadth of network. A carrier like WOW has expensive new planes; high lease/mortgage payments are a problem if you need to keep the planes on the ground due to lack of demand. FI, with their older fleet, can afford to reduce capacity and drop markets in the low season since the carrying costs of long-since depreciated aircraft are low.
College towns aren't a great option, either, because students aren't really making frequent trips to Europe (they don't have the funds and they're also supposed to be going to class most of the time). Sure, you can probably fill a plane or two with unidirectional traffic at the start or end of a break, but you're going to be flying empty in October or February.