Using old EV batteries in grid frequency control?

Has anyone thought about buying old EV batteries and connecting them to the grid for frequency regulation? Currently the FCR-N market in Finland is such that apparently you could make 30k€ a year with a 100kW setup. The average hourly price per MW has been ~36€/MW so far this year, so assuming you can participate without downtime the naive estimate would be

36€/MW * 24h * 365 * 0.1MW = ~31k€

This should be achievable on a 30k€ investment:

- 10k€ for the battery https://evshop.eu/en/batteries/292-2052-tesla-model-3-full-battery-pack-75kwh-long-range.html

- ~10k€ for inverter (Growatt WIT 100K-A)

- ~10k€ for grid connection + misc stuff

100% annual ROI. What am I missing?