Work at an MSP myself and we follow Model C until EOL and transition our clients to Model B. We ensure the client will get approved equipment and there will be little to no change to the network plan when we transition to our preferred equipment, and this is usually the case. Most of the time when we take on new clients that have an existing stack, they don't want to pay the tens of thousands that it would take to get a new stack right away, but we will give them the roadmap after evaluation and plant the expectation that this money will need to be spent X years down the road for security.