Not at all a wild bet. German electricity prices *routinely* swing by over 150€/MWh on a daily basis year round. That's a massive spread to make money off of.
For a good chunk of the year (April through to October), the prices even go negative at mid-day most days of the week. This will pay itself off very quickly
Why wild bet? It seems to be a safe bet that intermittent solar and wind availability will continue to create huge price swings, which storage can exploit and reduce to almost everyone's everyone's benefit. The investment just takes a while to break even, that's all. And with LFP or other long life battery technology, it will pay off for quite a while after break-even.
BESS asset life isn't great so a longer paypack period runs up against product life. Without running the actual model (volume, frequence and price differential) its tough to tell how quick it happens though I am sure I could build that relatively quickly if I wanted to (assuming granularity of public data).
My main point is that its a very large asset so you can't external forces come and mess up the financials (such as policy, regulatory changes, or large infra jump in that area) to make good on that bet. Certainly some public dollars being put to work to de-risk the bet.
For a good chunk of the year (April through to October), the prices even go negative at mid-day most days of the week. This will pay itself off very quickly