This is why the ulama have always advised us not to abandon nawafil prayers. They have observed that when someone gets relaxed about their nafil prayers, they will then get more relaxed about sunnah ghair muakkadah prayers, then sunnah muakkadah, then Witr. Ultimately, the quality of their fardh salah will suffer.
Allah condemns such people in the Holy Qur’an who perform their salah lazily:
إِنَّ الْمُنَافِقِينَ يُخَادِعُونَ اللَّهَ وَهُوَ خَادِعُهُمْ وَإِذَا قَامُوا
إِلَى الصَّلَاةِ قَامُوا كُسَالَىٰ يُرَاءُونَ النَّاسَ وَلَا يَذْكُرُونَ اللَّهَ إِلَّا قَلِيلًا
Verily the hypocrites would beguile Allah, whereas it is He who beguileth them, ; and when they stand up to prayer, they stand up languidly, making a show to the people, and they remember not Allah but little taqwa [an Nisa 4/1142]
May Allah protect us from complacency in our prayers.
[This short excerpt is based on a lecture delivered by Shaykh Riyadh ul Haq on 9th January 2017].