Secret ballots were devised in response to people historically doing things like this. Then they stopped happening because the countermeasures worked. If you take away the countermeasures, what then? Have criminals ceased to exist? Are the incentives somehow different now?
Three states exclusively use mail in ballots (starting with Oregon in 2000 I believe so at least 20 years), and if there is epidemic of criminal kneecapping going on, I feel like we'd have heard about it by now.
BTW, I believe Oregon started VBM in 1996.
Looks like they were voting on senators in 96, passed a measure in 98, and had their first all mail vote in 2000.