So much Trish skepticism all of the sudden! I feel like that was prime for eps. 3-4 but she's earned a lot of respect back since, hasn't she? ;)
Truthfully though, while Kev and Trish are at the heart of the show now, and I would hope for at least one more season... the show could definitely go on without them. Dennis's story and redemption has been as important to the overall arc as theirs; down the line, should Jamie and Jodie decide they want to move onto other projects, I could see Taron becoming the defacto lead easily.
(It's not unlike what happened on Shameless UK; Anne Marie Duff and James McAvoy left the show after Series 2, and TBH I really felt like the show wasn't the same without Fiona and Steve but it did continue to work for a few more series after that. It's gone on way too long now! ;) )