"is the using catalyst switches dangrous in terms of security threats such as unallowable login that couses denial-of-service or watchdog crash and othe cve that reported in cisco website?"
Yes it is, however often many don't understand it's not so much a question of dangerous but how much dangerous.
In the case of a no longer supported IOS, it's difficult to even really know if you're at higher risk until a security flaw is found in current software which also applies to older software too.
Even if a non-fixed security flaw is identified, what's the risk that someone will use it against you?
"is it making problem for our network security and are we forced to migrate to c9000 and small business switches?'
Are you forced? No. However, it can be very difficult to scientifically analyze the situation because you end up with probabilities not certainties. For example, you might run your existing equipment for another 20 years, without a security exploitation or be running the latest and best and be hit by a zero day exploit.
So, must you upgrade? Not really.
So, should you upgrade? Cannot say. That's something you need to decide.
I will say, much like insurance vendors recommend (buying) as much insurance as possible, hardware vendors also recommend (buying) newer equipment too.
