Would it be possible to just seal the interior of the boat with epoxy and dispense with the layer of glass?

Once you do a stripper and cut out hatches, you will see the strength the glass gives the substrate being on both sides of the strips when you feel the stiffness of the cutouts. The glass stops the strips from being able to bend apart. That's the more important characteristic that the glass brings vs. the notion that the glass is abrasive proof. If you take a scrap of glass fabric, try to rip it - you can't. Long answer to the real answer which is "no", don't just use epoxy on the inside.