Someone once said 'Never put down to enemy action anything that can be adequately explained as incompetence'.

The statement 'SQL Server is dying, the future is open source' seems a bit short of logical reasoning. Even if the statement had been 'SQL Server is expensive, the future direction is to use MariaDB' there may (just) be a logical reason, but the statement still lacks rigour.

If your manager's main concern is to reduce costs, the strategy is likely to be effective. Those experienced people with good skills (and good domain knowledge) will find other jobs, and new people with lower level of skills (and no domain knowledge) attracted by the lower salaries on offer will replace them. The end result is unlikely to help the business.

My suggestion is to try to engage with your managers. Organise some meetings where the issues they see can be aired, and their suggestions for the future discussed. Any competent manager concerned to give a high quality service to the business should welcome this type of discussion.

If everybody in the team understands the problems and supports a solution that both deals with the problems and improves what can be delivered to the business, then people can continue to have pride and confidence in the work they are doing.