Nothing wrong with doing what you love.
I don't think that studying two or more arts is what a lot of people are against, but rather not getting a good, strong base in everything and trying to absorb everything.
That's not the main issue either. What I believe is the main issue are the people who do not understand cross-training at all (usually people who want to learn how to fight after watching the UFC or something) and try to absorb what they can and move on, like a virus, and copy everything. For instance, they absorb one year's worth of material from wing chun, then quit, move to wrestling, quit, and move on continuing the cycle. That's not going to work. One reason is they won't truly understand it, just know the movement and they won't have enough experience with it to do anything. And it gets much harder if you try to copy and move with internal arts, because you have to understand the principles, the concepts, and the technique and you must have lots of experience. There is no quick fix. You possibly can't take in even a fraction of a system if you do it like that, and if you train like that, almost NOTHING will be useful.
Better to get a strong base (I say 5 years of HARD training), then try supplementing with what works best for you. For instance, even sport TKD can be combined with judo, wrestling, and hapkido easily. TKD can work with muay thai well, and visa versa. Boxing and TKD can work since most McDojangs don't know what the hell the fist is for.
Since you are not training under kenpo, tai chi, and TKD like a virus, then I encourage you to continue in those systems. I recently took up taiji again (had very little experience in wu taijiquan before as in a couple of weeks). Actually, I'm taking tai chi, not for combat but just for relaxation, recreation, etc. Almost nobody can take the punishment of hard arts forever, so at least some internal system is a must if you want to defend yourself when you're older. And one must feel comfortable (not saying you must be an expert) in all ranges and types of combat; the outside, in-fighting, grappling, on the ground, one person on the ground, weapons, weapons vs. hand, etc.) with a couple of areas of expertise. I'm a much better striker long-range/medium and grappler than close in-fighting with the elbows and knees, although I practice with my elbows and knees all the time, but it isn't my favorite position to be in. One has to be balanced yet have his areas of expertise. And by expertise, I do not a mastering techniques, which is almost impossible, but what you're best at.