Hell, I'd go so far as to say that the Sounder should be running, all the time, from Stanwood to Seattle, with a stop in Marysville. Also, expand operational times to outside commute times, and add holiday runs. If they did that, I'd be much more inclined to go to Seattle to "hang out".
The ridership on the "North Line" just isn't there to support pouring more money into it. (The Mukilteo station improvements are somewhat of an exception, since it's part of a multimodal project with the ferry terminal.) The South Line packs their trains, and there's some talk of running midday service there if they can afford to purchase slots, but no such talk north of Seattle.
The South Line has the advantage of being a fast, relatively direct route for the Green River Valley, since the buses take a while to make the east-west connection to I-5. Ridership for Tacoma isn't as high, given the relatively direct buses from downtown and the Dome TC to Seattle.
The North Line has the reverse, with buses taking a fast, more direct route that ultimately ends up being a shorter commute for anyone who doesn't work at Union Station. The only possible time advantages for Sounder over express buses would lie north/east of Everett because of two key bottlenecks (I-5 and the US 2/Hewitt Avenue Trestle).
The south corridor could adopt some of the bus restructures that the north went through, mainly consolation of off-peak runs into a single, more frequent route (512) while retaining peak hour routes (510/511/513).
The problem with the North Route is that it runs along the coast. Most of the people North of Seattle live closer the I-5 and see the drive to Edmonds, Downtown Everett, or Mukliteo as either too far, or a major inconvenience. If only the rail line that used to follow close by to I-5 had remained past the 70s (and currently part of the inner-urban trail, formerly the inner-urban rail) and could have been utilized, ridership would be much higher.
I will also add that some of the issues in the North have to deal with the sheer number of persons who work at Boeing, rather than downtown, coupled with people, at least from Whidbey, who purposely don't work 9-5 schedules to avoid traffic, and where a later series of trains (as well as more than one NB train in the morning) would be beneficial. ST should actually study the need and desires of those they are supposed to serve, and not assume that what works south of Seattle can or will work north of Seattle.
As someone who does utilize transit when they can, I can say that there are also some issues with route planning, making it appear as though it is much more difficult to get around (at least in the north) than it really is. There is no easy way to get from Mukilteo to Seattle or back, unless you take the Sounder, or actually look at the schedules on 3 different sites, since ST Bus isn't in Google Maps (only the Sounder), CT is only on CT's site or ST's site, and ET is only on ET's site. So if you, like me, just use Google for your trip planning, because you were on vacation in Portland where everything is tied in together, and they only have one transit system, you could be forgiven for thinking you need a taxi or wait 4 hours for a train to get home, when in fact, you can take a bus to Everett Station, then transfer to an ET for 75¢ to get to your ferry, instead of $2+ on CT, which will take twice as long. It is annoying.