It would have been... irritating. Season five ended with a literal bang - the detonation of the nuclear warhead. Faraday's theory was that detonating the bomb would change the timeline so that Oceanic 815 would never crash on the island. Had "Lost" abruptly ended after season 5, the audience would be left to assume that perhaps Faraday's plan worked. Oceanic 815 landed in LAX. It never crashed. The events that the audience had witnessed over the past five seasons no longer mattered. The entire series is effectively erased. And we never got to understand who Jacob was, who the MiB was, why these people were brought to the island, why the island was important and worth protecting, etc. etc.
I'm much happier with the way the series actually ended.