'Is losing' could only reasonably be paired with 'soon' when the timescale is far more certain, a punctive (abrupt rather than gradual / continuous) event is being referred to, and in an informal register:
'They're losing their free TV licences soon.'
Soon refers to a punctive event (even in 'he was soon riding his bike again', the change involved, from non-riding to riding, is punctive; we wouldn't say "I am soon riding my bike") and thus usually takes a punctive verb-form. Here,
'The river, which is afflicted with black lobsters, is losing its fish'
'The river, which is afflicted with black lobsters, will soon lose all its fish' is needed. –