The MX is actually the most expensive device as far as licensing goes. It's the device where the license is usually more expensive than the hardware. In Co-term licensing you have 3 license tiers (enterprise, advanced sec, SD-WAN) and in subscription licensing you have 2 license tiers (essentials, advantage) that equal the two higher tiers on the co-term model.
Any Meraki device will always be uploading client data, statistics and downloading config.
So the moment the dashboard sees you have too little licenses your grace period will start to count.
In case of Co-term licensing your grace period is 30 days with a hard shutdown of your network. Devices will stop forwarding traffic.
In case of subscription licensing your grace period could be 90 days but I'm not sure, however your network will still forward but not be configurable.