The early morning flight to Amsterdam is very busy now. Most of those people are using Amsterdam as a hub, it's not their final destination. Three example - you could leave Teesside at 06.20 and be in Los Angeles, San Francisco or New York by midday local time.
Travelling from Newcastle or Leeds or Manchester is a huge extra time penalty, subject to the vagaries of the weather. If you fly from any of those three, you may need to stay in a hotel overnight, so any cost saving on the ticket is lost, and more. And Newcastle and Leeds tend to be spoke airports to other hubs anyway, so the time advantage is gone.
If people have strong ideological beliefs stopping them from using the airport, then that's their choice. Actively discouraging others from using it is really petty though.
If I'm flying to the US I usually go from Manchester, and travel to there on the train, it's a breeze, and should be even better when they sort out the rail link, which is in process. It's like 2 Hours to Manchester (direct into the airport station), or an hours drive to Newcastle, this is not a huge penalty, especially not for 10-100x more flights/ time options and more facilities, never mind more holiday package companies.
It doesn't seem to make any sense to me to fly for an hour in wrong direction to Schipol, wait there 1-5 hours and then fly 1 hour back, to get over the UK to get approximately where they started from. This being cheaper would also be extremely rare.
You can go from Newcastle or Manchester to San Fran via Heathrow in 14 hours, for £500, to go from Teesside it's more like 18 hours, 2 more stops and another £100. It's hard to make that make sense. The only way you would need a hotel the night before to fly from Manchester (which I've done a few times) is if you were tyring to get a super early flight to save money, so you might save 200-500 on your holiday, for the cost of a £100 hotel room at the clayton (which includes parking, if you want to drive).
Going to Europe anywhere which isn't a direct flight would be hard to make sense going via Amsterdam, and if going to the far east then one of the other 20 airlines will be 20% cheaper (and probably a bit better than KLM).
I don't have ideological beliefs about using the airport, and neither does skyscanner, in my ideal world I would fly everywhere from there as it's 5 minutes from me, but from about 10 trips in the last 5 years it's worked for one, a trip to Amsterdam.
All I'm bothered about is total trip time door to door and cost. I want it to make more sense, but I don't see how it largely can until it has a load more flights. I don't see how long haul would work well from there, as it's not a long haul airport, so best hope for it is a lot more EU flights to common EU summer sun, and winter sun destinations. Or maybe even more internal flights to LHR or Manchester which are basically like cheap busses. This may work when short haul air travel can be electric or cheaper powered.
I wouldn't advise anyone from not using it, never, quite the opposite, but I would advise people to do what makes the most sense for them, which is usually a shorter trip door to door, more options and less costs (from wherever that is). I just wouldn't advise people use something because it's local, trying to give the impression it's better than it might be, and the "support the local airport" line won't keep the airport afloat.
I want it to work, much larger scale, but it's a tough ask.