Naw, specifically in your case you will have no issues with part availability because it really is just a base package G37 with a smaller engine and in most cases will find it considerably cheaper to maintain than other cars still in production.
Per the wiki:
Priced lower than the G37, the G25 was aimed to compete with entry-level luxury rivals such as the Lexus IS250, Audi A4 and BMW 328i. Compared to the G37, other than a smaller engine and fewer options (no six-speed manual transmission available, no Navigation, no Premium or Technology packages in the U.S. market), the G25 was otherwise identical and features a choice of rear-wheel drive or ATTESA all-wheel drive.
Normally you might find that because it had a limited run engine the parts we're more expensive and harder to find but its the same engine as the M25 and EX 25 which increases the volume of cars sold and therefore parts produced.
More generally almost every 10 year old car that was sold in the US but is no longer made will still have parts available for it (due to US law).
Edit: It may actually be 8 years for parts. I would have to go find the specific set of laws and court cases regarding consumer protections that spawned it to be sure.