When the value of the try is increased, there is a concomitant increase in value of stopping that try.
Teams already do everything to try and stop tries. It would make a negligible difference on the defensive team. If anything, they'd have to be more disciplined. It's easier now for the defensive team because they know if they give away a penalty the attacking team will kick for points. They might then take a while to get hot on attack again. It usually takes a few penalties close to the line to get on a yellow card warning, so they're safe to give away a few knowing they'll concede a shot at 3 points, and will be able to relieve the pressure without conceding 5 or 7.
If teams were encouraged to keep attacking, then they wouldn't be able to keep giving away penalties as they'll get yellow cards much sooner.
When the varsity cup introduced 2 point penalty goals and 3 point conversions there was only a minor increase in the number of penalties (something like 5% - i.e. about 1 per game, yet a big decrease in penalty goals). I would also bet there was an increase in the ball in play time, so relatively it probably stayed about the same.
If you devalue, or even get rid of penalty goals all you have to do is use yellow cards sooner for repeated or cynical infringements. You don't need to make it any more complicated IMO.