"can you tell me more on the disadvantage we might have on using old chip"

It may not be possible to buy in the required volume - if at all!
Even if you can find some today, how long will they continue to be available?

Similarly, it may require obsolete support components.

You might not be able to get surface-mount package (dunno if that matters to you); if you can, and they're old stock, there can be issues with moisture absorbtion.

It may not be compatible with modern requirements of low power, 3V interfacing, etc, etc,...