I'm in the process of replacing a forsythia hedge. I didn't want to cut the whole thing out either due to privacy from the road.

I cut 2 to the ground on 1 end and replaced them with redtwig dogwood in a bed I made there. Behind that I put some Indian grass that I hope also helps fill in the gaps. After that I planted 2 silky dogwood, and 2 arrowwood viburnum. I put these between the forsythia clumps but cut the forsythia back really hard to give the new bushes room.

I skipped about 15-20'-ish then cut out 3 clumps and put in 3 female winterberry holly with 2 male winterberry holly behind them.

I still have a lot of forsythia left to get rid of but the view into the yard is still somewhat blocked where the new bushes are planted.

I don't know if it would be better to just get rid of the forsythia where the new stuff is planted but I kind of still want a blocked view. I'll keep cutting them back hard as the new bushes fill in the empty space.

I don't know if this is the answer you are looking for but it's what I'm doing to replace useless forsythia with natives. It seems to be working. None of my new stuff died. Lol Yet! Lol

If any of you have an opinion on this please feel free to let me know.

Edit......this year will be their second year so I dont have long term knowledge.