The value is retained.  The miner is forced to sell to cover expenses, this constant sell pressure drives the price down, they have little choice in this matter, OTOH the masternode owner is not forced to sell and can hold onto those coins keeping them off market.  In practice though, this does not happen because the mood on DASH is bearish and so the masternode owners sell the coins miserably, also we have a large number of masternodes run by exchanges using customer funds, eg Binance and they sell the rewards as additional profit for themselves.

One of the things we can do to improve the mood is the pay more to the masternode owners, in doing so it would make the endeavour of running a mnode more enticing and draw in more investment which would lift the price and improve the mood which would then cause fewer MNOs to sell their rewards, since nothing is forcing them and the price would rise ever more.

However, improving the mood in DASH is very difficult, DCG are tone deaf to the environment and as a software development company they sure do suck at delivering software, Evolution is still nowhere on the horizon, all we know for sure is it won't be delivered this year.  The core wallet team are unable to even keep up with the Bitcoin backports as we are now 2 years behind Bitcoin, their new head of marketing seems unable to bust a move and they seem intently focused on the market in Venezuela which has proven to be a non-starter.

Add to this the competition in the space from DeFi and innovations like HEX which is now the third ranked coin by market cap. and it is little wonder DASH finds itself where it is, sadly without material change in DCG we can expect more the same going forward.