Would it be difficult for a country with multiple borders (eg France/Germany) to "leave" Schengen, but maybe easier for Spain/Portugal as there's just that one (and mountainous) land border with France?
From a purely "is it possible?" point of view, it's pretty easy to do it. If you look at the French/Belgian border, you can see that the vast majority of crossings are just tiny country roads that can be easily barricaded and traffic rerouted elsewhere. Schengen doesn't require fences or other physical barriers, so it's just a matter of putting a few concrete blocks down and adding some signs.
The biggest issue is the lack of infrastructure to implement border controls: most of the former facilities have either been sold or demolished. It's not such an issue on motorways and dual carriageways, especially as most of the former checkpoints were turned into parking areas and they can easily be reactivated with a few modular buildings. Motorways like the A16 can easily be reconfigured to hold a modern border checkpoint, especially as intra-EU border crossings have no need for large truck infrastructure for customs controls.
But in urban areas, there simply isn't the space to build anything resembling a modern Schengen checkpoint. These urban border crossings pre-1995 were very informal in most cases, with people simply being waved through if an EU passport/ID card was presented. Now, all identity documents have to be scanned as part of the 'minimal check' by the Schengen state, so even if France didn't care about scanning anyone, Belgium would still need to do it. It's not impossible, but many of these urban towns have essentially become one town, built up to the border and redesigned to remove any trace of the old border facilities.
For example, take the twin town of Wervix-Wervicq-Sud. The border checkpoint used to be on the Belgian side, which then became a rather derelict wasteland with informal car parking. At some point in the 2010s, they built a new bridge, and the entire Belgian side of the river was renovated. To place a new border crossing there would require tearing up all of the landscaping that has been done: not impossible, but it would require extensive road reconfiguration and rebuilding just to be able to fit enough lanes there to cope with the modern amount of traffic.
But from a purely "would they do it?" point of view, the general perspective of the EU is that no-one is particularly bothered about non-EU travellers having to deal with the EES. From everything that I've read, the biggest issue seems to be that there isn't a single method for dealing with the EES, so some countries are requiring the use of the kiosks on every visit, whereas others are allowing already-registered travellers to go straight to the e-gates or manual controls. It seems to be causing widespread confusion, and I think the most sensible approach would be to put the registration kiosks in as many departure airports as possible, then make it clear that if you're already registered, you can proceed straight to the e-gates or manual control. So, someone leaving from Stansted should be able to register for the EES, and then on arrival, they wouldn't touch a kiosk, only the biometric scan at the actual gate.