If they want to maintain their performance next season then Leverkusen should try to add depth to their squad because they will be playing in the Champions League. The decision to stay is much better than leaving too quickly to take charge of another team because Xabi Alonso needs to prove his quality as a coach. Playing in the Champions League will mean that Xabi needs several quality players because the matches there will be much different from the Bundesliga.
Next season will be a real challenge for Leverkusen. They will play in the Champions League and that is a high level competition. Xabi must be able to rotate players and add variations to match tactics. I think club management will give Xabi a budget to find the players he wants. And for tactics I think if you play at a level like the Champions League you really need a variety of tactics because strong teams from other leagues will look for your weaknesses. And if they find your weaknesses then the teams in the domestic league will also find out your tactical weaknesses if you lack tactics.
The next challenge is that Bayern Munich will be better prepared for Leverkusen. I'm sure this season they really didn't expect Leverkusen to be a problem for them. And they definitely won't let that happen again next season.
Your remark about Leverkusen's issues is valid. Champions League is a different world, and Xabi encounters challenges. This summer matters. Losing important players would be devastating as big clubs approach. We all know that game. It could ripple, thats the risk. We shouldnt rule them out yet. They must find stability under this uncertainty. It'll be crucial for the Bundesliga and Champions League.
Bayern Munich stumbled this season, but they always recover. Analyze, learn, modify. Leverkusen must plan and outwit them. A vibrant, well-oiled team is more important than large signings. Xabi's test is big, but Im optimistic. The beauty of sports is that underdogs can win.