BitTorrent (BTT) price, performance and general information

This question is about the BitTorrent protocol:

A BitTorrent .torrent file contains a list of trackers. For example:

• udp://tracker.torrent.eu.org:451
• udp://tracker.coppersurfer.tk:6969

What happens when those trackers are not available (i.e. they go offline)? Does it mean that the files can no longer be downloaded?

I am aware of the DHT, which is supported by some clients. So my second question is: with DHT, what is the use of listing the trackers in the first place? (I mean, there is no guarantee that the trackers listed in the .torrent file would still be around in 10 years, so why bother listing them at all?)

https://it.slashdot.org/story/19/05/13/2255229/academics-improve-sha-1-collision-attack-make-it-actually-dangerous

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Just wondering if this can be used against bittorrent?

I don't want to manually go through & compare 50+ files to see which ones I "need".

Is there a client with settings / a plugin to not replace existing files but only download what isn't already found ?