Is there any good reason not to use IPv4 only within a local network?

We have a small office with maybe 10-20 devices connected to our local network (LAN). We are running an internal DNS for resolving local network domains. The DNS has been configured with only IPv4 addresses, because it was easier to set up like this, and there seems to be no reason to use IPv6 addresses for our local network devices. None of the devices needs to be reached from outside (WAN).

Am I overlooking any good reason to use IPv6 within the local network?

Note that this question seems to be a duplicate of Why would you use IPv6 internally? only at the first glance. My question is about using IPv4 internally, while the other question discusses NAT etc. which doesn't apply here.

None of the devices needs to be reached from outside (WAN).

Yet.

Some day you merge with a different company, who also uses the 10.0.1.0/24 block, and they want to integrate your network with theirs. Things turn ugly, and everyone asks why RFC1918-addresses were used.

The IPv6 ULA is big enough that you can pick a range, and be reasonably sure that noone else will use the same range, if you pick a random /64 or four.

In your case, there is no practical benefit to use IPV6. IPV4 will serve all your requirements.

However, if you want to be ready for future ahead of time, you can play with IPV6. But it may be another decade or two when IPV6 becomes more prevalent.