So, it's been over three years since the last chapter? No official update from the publisher or the mangaka? The best way of looking at it is no news is good news. If it had been cancelled, we would've heard by now. It could be that Takaya is burnt out, like Takehiko Inoue with the manga Vagabond (which has been on indefinite hiatus longer than Guyver). It would not hurt for someone to come out and explain the situation. I guess it's just not the Japanese way of doing things.
At the end of the day, The Guyver is their intellectual property, and if they want to call it a day, we can't do anything about it. It would be a crying shame for all the people who are invested in the series, as the story deserves closure.