Xabi is taking risk by staying in Leverkusen as they might not be able to repeat this success next season. With limited player resource, they will have big troubles by defending their Bundesliga title while discover Champions League. It will put Leverkusen under pressure of using their player resource very wisely to maintain good performances in Bundesliga but still don't affect their discovery about their own ability in the Champions League.
I think staying at Leverkusen is the lowest risk of them all. Going to any of the teams coming for him would be a very big risk. Can you imagine the pressure that'll come with coaching a club like Madrid or Bayern or even Liverpool?
It's a very good decision to stay at Leverkusen for at least one more season. He just won them the league and took them to the Champions League, it won't be proper to not play the Champions League with them before going to a bigger club.
There is less pressure at Leverkusen. He has won them the league, if they finish next season without a trophy it wouldn't mean anything for a club like Leverkusen. What matters is he brought them to the Champions League and they will compete for it. Who knows, they might surprise us again.
Winning the league will open up doors for better recruitment, so they'll be able to recruit better players. Also, with Alonso staying, the star players of Leverkusen won't leave. When a team has a good project like Leverkusen does, convincing players to sign for them won't be a problem.