In my fantasy world, Delaware would be tolling all traffic on the Delaware Turnpike part of I-95 - from the mainline barrier between the state line and DE-896, as far east (north) as present-day Exit 4 at DE-7.
Do keep in mind that the original plan for the Delaware Turnpike portion of I-95 was to have all the tolls eliminated once the original bonds that initially built the road were paid off.
I do not remember there being any tolls on the ramps at DE-141 or beyond.
By beyond I'm assuming that you mean north or east of the DE 141 interchange. There's good reason for that. I-95 north of there is not part of the Delaware Turnpike system.
Anyway if Delaware was serious about reducing traffic exiting and bypassing the tollbooths (which are only there because of that portion of I-95 is the Delaware Turnpike and not due to waterway crossing); they could rationalize (i.e. lower) that extortion-like $4 toll and/or reinstate E-ZPass discounts. While doing such would mean that less money would be collected per vehicle; the overall revenue collected could be higher and the local roads won't be as flooded with through-traffic as it is now.
How much traffic is actually diverting from I-95 that would've continued on I-95?
Once you get off the exit, there doesn't seem to be any real congestion issues caused by those on 896 North who eventually go to 279. Likewise, those on 896 South that go to Old Baltimore Pike appear to be headed towards Elkton, and it's doubtful they would remain on 95 regardless of the toll since the time savings is minimal...or would even cause them to backtrack. Traffic levels on 279 in Maryland always appear to be light. Traffic levels on 896 in Delaware always appear to be in line with traffic on 273 around 95, which isn't a shunpike route.
There's always plenty of talk on here as if half the traffic on 95 exits to avoid the toll, but that's not really the case. And there's people on here that have stated they'll always avoid that toll plaza regardless of the toll.
Even if 10% of the traffic detours off I-95 to simply continue to I-95, if the goal is to keep every single one of those drivers on 95 it would mean tolls could only fall about 10% to $3.60 in order to maintain the status quo. Since most people that avoid the toll now will probably continue to do so, it'll result in a net loss for Delaware.
Do some people use US 40 to avoid the I-95 toll? Probably. I can't imagine too many of them jumping over to I-95 willing to pay $3 compared to $4. I think, as discussed here, many people use US 40 to avoid the traffic on I-95. If reducing the toll on I-95 is done to encourage people to stay on I-95, motorists aren't going to suddenly pay a reduced toll if there's more traffic.