We just had an election in Alabama that proved this view incorrect. Even in seemingly "safe" districts your vote can matter. That's how safe districts become safe.

Howard Dean pursued an explicit "fifty-state strategy" as chairman of the Democratic National Committee in the mid-2000s, putting resources into building a Democratic Party presence even where Democrats had been thought unlikely to win federal positions, in hopes that getting Democrats elected to local and state positions, and increasing awareness of Democrats in previously conceded areas, would result in growing successes in future elections.

The strategy was gradually abandoned after Dean stepped down from the DNC, and I believe that a large part of the Democrats' losses since then is exactly a result of your mindset, since abandoning red states or districts as lost causes only allowed the Republican Party to grow even stronger in areas where it was unchallenged, resulting in lopsided losses for Democrats in even more races and killing any ability to lay the groundwork for future victories.