Similarly, there's nothing in C++ that prevents it from taking C code (I could be way wrong on this, I don't use C++).
Indeed, that's mostly wrong. C and C++ are definitely two different languages with a different set of rules.
Unless you write C that *complies* with C++ rules, it won't compile as C++. A typical C project would need a lot of adjustments to compile as C++. Not worth it.
So, I like simple languages.
Simple is good. Wirth agrees. As simple as possible, but not simpler!
Multiple inheritance, for instance, is a plague that many serious computer scientists have asked to avoid at all costs.
OO-programming is pretty old stuff actually. It became popular as a hype, but has existed long before it became popular. Unfortunately, the whole hype around it pushed the release of atrociously complex languages that went way too far to solve inexistent problems by creating new ones.
So to the language inventors of the world: Keep It Simple - Stupid! The KISS principal. Don't create the next DEBE language (Does Everything But Eat).
Keeping things simple yet useful and consistent is a tough thing. That's been Wirth's battle for most of his life.
Popularity is a special beast as well. C has been popular because it's simple enough (thus simple to code for, and write compilers for...), does the job and has been well thought out. C++ became popular (even though maybe not as much) because it was the first language supporting OO paradigms that was at the same time almost as low-level and flexible as C (thus piggy-backing on C's merits). Java became popular mainly thanks to its "run anywhere" promise.
All in all, for a language to succeed, it has to solve problems that could not be solved before, or at least a lot less conveniently.
Many attempts at creating the next PL have failed mainly, IMO, because they didn't really solve anything that hadn't been solved already. Or because they were way too complex to handle. Or often both.
Some attempts should have had better luck though... but they were probably there either too early or too late.
Take a look at Modula-3 for instance. A serious look.