It’s always sad to see crosses being removed, but this is different. The building was sold and is being repurposed. It’s up to the new owner what they wish to show on the outside. Perhaps some Christian Churches can buy the crosses to display elsewhere. This is not necessarily another example of Muslim-Creep and encroachment.
Once the building is sold (circumstances and conditions will vary depending on the city, the "historic landmark" designation, etc.), then it's nothing but a building.
The real tragedy is all the beautiful and fantastic old historic churches that are everywhere...like "century homes", the plain upkeep and maintenance costs can be astronomical. Also for "historic" downtown/city buildings.
It's very sad to see historical and heritage sites keep going down the drain, nevertheless. It's also a little disturbing to me to see a Catholic Church building being converted into a Mosque, but that's just my personal opinion.