Bal Tigra and Bal Tosif Because Law Codes Devolve

An interesting idea by Shadal, into the reason for Bal Tigra and Bal Tosif, as explained by the verses themselves.

ב ] לא תוסיפו : צריך שתשמעו אל החוקים והמשפטים האלה לא בלבד לעשותם , כי גם לעשותם בלי תוספת ומגרעת , וכל זה מפני שהרבה מתועבות העכו"ם נמשכו מעט מעט מן המנהגים הקדמונים אשר היו בעצמם ישרים ; וע ' למטה י"ב כ"ט , ל , ל"א י"ג א

That is, even the early practices of the gentiles were good - yesharim. But slowly, little by little, they devolved into the abominable idolatrous practices. Thus, these two concepts are connected. And he brings proof to this from later on, in Devarim 12:

כט כִּי-יַכְרִית יְהוָה אֱלֹהֶיךָ אֶת-הַגּוֹיִם, אֲשֶׁר אַתָּה בָא-שָׁמָּה לָרֶשֶׁת אוֹתָם--מִפָּנֶיךָ; וְיָרַשְׁתָּ אֹתָם, וְיָשַׁבְתָּ בְּאַרְצָם.	29 When the LORD thy God shall cut off the nations from before thee, whither thou goest in to dispossess them, and thou dispossessest them, and dwellest in their land;
ל הִשָּׁמֶר לְךָ, פֶּן-תִּנָּקֵשׁ אַחֲרֵיהֶם, אַחֲרֵי, הִשָּׁמְדָם מִפָּנֶיךָ; וּפֶן-תִּדְרֹשׁ לֵאלֹהֵיהֶם לֵאמֹר, אֵיכָה יַעַבְדוּ הַגּוֹיִם הָאֵלֶּה אֶת-אֱלֹהֵיהֶם, וְאֶעֱשֶׂה-כֵּן, גַּם-אָנִי.	30 take heed to thyself that thou be not ensnared to follow them, after that they are destroyed from before thee; and that thou inquire not after their gods, saying: 'How used these nations to serve their gods? even so will I do likewise.'
לא לֹא-תַעֲשֶׂה כֵן, לַיהוָה אֱלֹהֶיךָ: כִּי כָל-תּוֹעֲבַת יְהוָה אֲשֶׁר שָׂנֵא, עָשׂוּ לֵאלֹהֵיהֶם--כִּי גַם אֶת-בְּנֵיהֶם וְאֶת-בְּנֹתֵיהֶם, יִשְׂרְפוּ בָאֵשׁ לֵאלֹהֵיהֶם.	31 Thou shalt not do so unto the LORD thy God; for every abomination to the LORD, which He hateth, have they done unto their gods; for even their sons and their daughters do they burn in the fire to their gods.

leading into perek 13:

א אֵת כָּל-הַדָּבָר, אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם--אֹתוֹ תִשְׁמְרוּ, לַעֲשׂוֹת: לֹא-תֹסֵף עָלָיו, וְלֹא תִגְרַע מִמֶּנּוּ. {פ}

1 All this word which I command you, that shall ye observe to do; thou shalt not add thereto, nor diminish from it. {P}

This idea of gradual transformation of a religion, little by little, is an interesting one. I wonder how much this in fact has been the case in Judaism -- via takana and minhag -- the halachically permitted forms of bal tigra and bal tosif. And whether this transformation is good -- as surely much of it is.