As long as this man had some kind of unique resources, it wouldn't hurt to waste just one hundred years to be this man's contracted beast.
Don't look at how fast Cellino grew in the past. If not, because he lived in the magic prairie since he was still a cub and he also lived around the world tree, no matter what, his growth would not be this fast.
It was normal for beasts, especially those who had turned into sacred beasts to spend hundreds of years just to advance slightly.
That's why, it was already an unwritten rule that sacred beasts wouldn't kill each other and would always leave a leeway to their 'enemies', no matter how deep the grudge was.
Once they were found to be killing their peers, the other sacred beasts would definitely hunt them down and eliminate such an unstable factor.