The MX250 has not had an end of life announced so far, regarding the AI issue. The vast majority of MX models have the same features, what will change is a matter of capacity.
I have no idea how Meraki is treating this issue today, but if you are thinking about security issues, just remember that security goes far beyond a firewall, and even though you have several security elements on your network (FW, IPS, WAF, MFA, etc.) none of this will guarantee 100% security. Technology is constantly evolving and as a result, more and more vulnerabilities are emerging.
https://documentation.meraki.com/General_Administration/Other_Topics/Meraki_End-of-Life_(EOL)_Produc...
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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