I searched on the NS website and found that we can take bus #70 from Giethoorn to Steenwijk, and then take a train to Amsterdam. One of the routes includes a single transfer at Almere Centrum, where the change is on the same platform—from Track 1 to Track 2—with a 5-minute window to catch the connecting train to Amsterdam.
My question is: if we buy a train ticket from Steenwijk to Amsterdam and miss the connecting train at Almere Centrum, are we allowed to take the next available train, or would we need to purchase a new ticket?
You can take the next train and would not need to buy another ticket.
It is a cross platform connection, so you just walk 20 ft or so from one train to the other. 5 minutes is more than enough time.
But another reason not to worry is the Dutch train system works like a giant subway system. You can scan in with a credit card to enter Steenwijk station platforms, and scan out with that card in Amsterdam to exit the station. Now the system knows to charge your credit card for travel from Steenwijk to Amsterdam. You never exit the platform area at Almere Centrum, so the system does not care what whether you change trains or not, or what train you were actually on.
You also use your CC on the bus. So you tap your card when boarding the bus, then again when leaving it in Steenwijk, then again at the entrance to the platforms, then again in Amsterdam...
Sometimes there are gates, sometimes only totems. Don't know what Steenwijk has.
(General rule all over Europe: You never need to buy a new ticket if you miss a train through no fault of your own).