Sticking to plain C has the advantage of probably being the subset of C++ (I know, it’s not technically a subset) that’s in widest use.
I prefer using C++ like "C but with cin and cout" and I informally call it C+ :)
Is it not? I thought C++ was explicitly a strict superset of C?
More importantly, though - the syntax is not what matters. The languages are very very different in idiomatic use. You just don't write programs the same way with C and with C++. And the gulf between the two only expands with time. A decent C program is almost certainly a poor C++ program, in terms of idioms, utilization of library facilities, and sometimes even in terms of performance (!).
Look at for ex. what const means in each, or auto. Or variable length arrays (feature that doesn't exit in C++). Or designated initializers that C++ didn't have for a long time.
Here is a good list of differences: https://mcla.ug/blog/cpp-is-not-a-superset-of-c.html
There are plenty of legal C programs that aren't legal C++ - if for nothing else, then because C++ has more reserved keywords which are perfectly legal identifier names in C.
E.g. this is legal C but not legal C++:
int template = 10;