Xabi Alonso made an excellent decision to continue with the team next season. Leverkusen will play in the next Champions League, they will need a coach like Xabi Alonso because he has played in major competitions such as the Champions League. I can't wait to see the club compete with the clubs from other leagues, as we have seen how they competed with Bayern Munich for the Bundesliga title race this season. You're right, they need to add more players next season, so they shouldn't be eliminated so easily, because they will face difficult challenges in the next Champions League if they meet a major team like Real Madrid or Manchester City, we don't know if their current players will have strength to play this way next season.
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 Champions League.
They are both big tasks for Xabi Alonso and his players next season and I think Xabi decided to stay because he aims at a coach position in Real Madrid that is not available this summer. They will complete the contract with Carlo Ancelotti before finding a new coach in 2025 summer, not this coming summer next 3 months.
If by the end of the season Leverkusen is undefeated, that will be the best record for Leverkusen and that must be appreciated because Xabi has brought Leverkusen this season to become champions in the Bundesliga. I also won't be happy about him leaving Leverkusen because in my opinion, he deserves to bring Leverkusen to compete in Next season's UCL where of course their level of play must be above the domestic league, at least making it into the important round of the UCL is good enough, after all, if Xabi Alonso succeeds in bringing Leverkusen to the important round of the UCL it will make him even more famous and ogled by many clubs big.
I think all Leverkusen supporters don't want Xabi Alonso to leave this team, because what Xabi Alonso has built would be a shame if he just left it, so the most important thing is that Leverkusen deserves to enter a higher level and compete in the UCL, I'm sure they deserves to enter the UCL and compete with Munich to win the UCL title next season even though they have to meet other top European clubs which of course will be the latest challenge for Leverkusen which can give them a lot of experience in the UCL next season.